diff --git a/.forgejo/workflows/test.yml b/.forgejo/workflows/test.yml
index 4d5abdb..39d8ef1 100644
--- a/.forgejo/workflows/test.yml
+++ b/.forgejo/workflows/test.yml
@@ -38,3 +38,31 @@ jobs:
 
       - name: Run tests
         run: /tmp/venv/bin/python manage.py test
+
+  e2e:
+    # The browser tests (e2e/, docs/refactoring.md): a real runserver on its own
+    # throwaway database, a book uploaded, read, highlighted, the theme switched.
+    runs-on: ubuntu-latest
+    container:
+      image: catthehacker/ubuntu:act-24.04
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install Python dependencies
+        run: |
+          python -m venv /tmp/venv
+          /tmp/venv/bin/pip install --no-cache-dir -r requirements.txt
+
+      - name: Install Playwright and Chromium
+        run: |
+          npm ci
+          npx playwright install --with-deps chromium
+
+      - name: Browser tests
+        env:
+          CI: '1'
+          E2E_PYTHON: /tmp/venv/bin/python
+        run: |
+          mkdir -p data
+          npx playwright test
diff --git a/docs/refactoring.md b/docs/refactoring.md
new file mode 100644
index 0000000..16c22bf
--- /dev/null
+++ b/docs/refactoring.md
@@ -0,0 +1,52 @@
+# Refactoring (started 2026-10-14)
+
+diora grew fast across four code bases that share one web front end:
+diora-web (server + app.js and friends), diora-android (Kotlin + web-native/*.js),
+diora-desktop (Electron + desktop.js) and the vendored copies between them.
+This is the plan for tidying it up without breaking anything.
+
+## Rules
+
+* **No behaviour changes in a refactoring step.** Features and fixes go in their own commits.
+* **Tests before moving code.** A step only starts when what it touches is covered.
+* **Small steps, each one releasable**, each one checked on web, desktop and Android.
+* Whoever owns a file does its refactoring (web-native/* → the Android session;
+  app.js, home.js, templates, desktop → the desktop session); shared contracts live in docs/.
+
+## The numbers (2026-10-14)
+
+| file | lines |
+|---|---|
+| static/js/app.js | 6 864, about 275 functions, about 120 globals, 45 sections |
+| static/podcasts/native-podcast.js (copy from diora-android) | 4 554 |
+| static/css/app.css | 2 010 |
+| static/js/podcasts-web.js | 1 899 |
+| podcasts/views.py | 2 505 |
+| web-native/native.css | 2 150 |
+
+Web JavaScript has no unit tests and four Playwright smoke tests that run nowhere.
+The reading-position rules exist four times (books/views.py, app.js, desktop ops.js,
+Android Ops.kt), each tested separately or not at all.
+
+## Steps
+
+1. **Safety net** (in progress)
+   * Playwright tests that run locally and in CI against a throwaway database: log in, open a
+     book (an EPUB fixture), read, the position is saved and restored, a highlight, a note
+     in the margin, the theme switch, radio and podcasts tabs load.
+   * Shared test vectors for the reading-position rules (`docs/progress-cases.json`), run by
+     the Django tests, desktop's node tests and Android's OpsTest — one contract, checked
+     in all four places.
+2. **Split app.js** by its sections into plain scripts loaded in order (radio, focus,
+   ambient, crypto, epub, books, reading position, reader, tts, highlights, margin, …). They
+   stay classic scripts sharing globals, so the native layers that wrap `window.*` keep
+   working unchanged; template, sw.js SHELL and Android's build_web list them. No code
+   changes inside the moved functions.
+3. **Dead code**: features and helpers nothing calls any more (found by the split and by
+   coverage of the tests), removed one at a time.
+4. **Server**: podcasts/views.py into modules by area (feeds, episodes, queue, player sync,
+   import); shared helpers (`_require_auth`, JSON body parsing) in one place.
+5. **CSS**: the remaining fixed colours to the theme tokens (docs/theme.md); app.css split
+   like app.js.
+6. **Copies between repositories**: one sync script per direction with a check in CI that
+   the copy matches its source revision (today: sync_native.sh, sync_podcast_ui.py).
diff --git a/e2e/env.js b/e2e/env.js
index 67164b0..48fee64 100644
--- a/e2e/env.js
+++ b/e2e/env.js
@@ -1,6 +1,9 @@
 // Shared constants between playwright.config.js and e2e/global-setup.js.
-const PORT = 8000;
+// E2E_PORT / E2E_PYTHON: a port that no dev server holds (the suite must never
+// run against a real database), and the Python with the requirements installed.
+const PORT = Number(process.env.E2E_PORT || 8000);
 const BASE_URL = `http://127.0.0.1:${PORT}`;
+const PYTHON = process.env.E2E_PYTHON || 'python3';
 
 const SERVER_ENV = {
   ...process.env,
@@ -15,4 +18,4 @@ const TEST_USER = {
   password: 'e2e-test-pw-12345',
 };
 
-module.exports = { PORT, BASE_URL, SERVER_ENV, TEST_USER };
+module.exports = { PORT, BASE_URL, PYTHON, SERVER_ENV, TEST_USER };
diff --git a/e2e/global-setup.js b/e2e/global-setup.js
index 07e636d..cb36a8a 100644
--- a/e2e/global-setup.js
+++ b/e2e/global-setup.js
@@ -11,7 +11,7 @@ const fs = require('fs');
 const path = require('path');
 const { execFileSync } = require('child_process');
 const { chromium } = require('@playwright/test');
-const { BASE_URL, SERVER_ENV, TEST_USER } = require('./env');
+const { BASE_URL, PYTHON, SERVER_ENV, TEST_USER } = require('./env');
 
 const ROOT = path.join(__dirname, '..');
 const DB_PATH = path.join(ROOT, 'data', SERVER_ENV.DIORA_DB_NAME);
@@ -22,14 +22,14 @@ module.exports = async function globalSetup() {
   // Fresh DB every run.
   fs.rmSync(DB_PATH, { force: true });
 
-  execFileSync('python3', ['manage.py', 'migrate', '--noinput'], {
+  execFileSync(PYTHON, ['manage.py', 'migrate', '--noinput'], {
     cwd: ROOT,
     env: SERVER_ENV,
     stdio: 'inherit',
   });
 
   execFileSync(
-    'python3',
+    PYTHON,
     [
       'manage.py',
       'shell',
@@ -45,6 +45,8 @@ User.objects.create_user(${JSON.stringify(TEST_USER.username)}, "", ${JSON.strin
   );
 
   fs.mkdirSync(AUTH_DIR, { recursive: true });
+  // The book the reader tests read (e2e/make_epub.py).
+  execFileSync(PYTHON, [path.join(__dirname, 'make_epub.py'), path.join(AUTH_DIR, 'book.epub')], { stdio: 'inherit' });
 
   const browser = await chromium.launch();
   const page = await browser.newPage({ baseURL: BASE_URL });
diff --git a/e2e/make_epub.py b/e2e/make_epub.py
new file mode 100644
index 0000000..6f9d905
--- /dev/null
+++ b/e2e/make_epub.py
@@ -0,0 +1,58 @@
+"""A small EPUB for the e2e tests: 12 chapters of numbered paragraphs (long enough
+to scroll and to have a reading position), one footnote. Written to the path in
+argv[1]; e2e/global-setup.js calls it for every run."""
+import sys
+import zipfile
+
+CONTAINER = """<?xml version="1.0"?>
+<container version="1.0" xmlns="urn:oasis:names:tc:opendocument:xmlns:container">
+  <rootfiles><rootfile full-path="OEBPS/content.opf" media-type="application/oebps-package+xml"/></rootfiles>
+</container>"""
+
+
+def chapter(n):
+    paras = []
+    for p in range(1, 41):
+        text = f'Kapitel {n}, Absatz {p}. ' + ' '.join(
+            f'Wort{w} im Absatz {p} des Kapitels {n} steht hier, damit der Absatz einige Zeilen lang wird.'
+            for w in range(1, 6))
+        if n == 1 and p == 3:
+            text += ' Mit Fußnote<a href="ch12.xhtml#fn1" epub:type="noteref"><sup>1</sup></a>.'
+        paras.append(f'<p id="c{n}p{p}">{text}</p>')
+    notes = ''
+    if n == 12:
+        notes = '<aside epub:type="footnote" id="fn1"><p>1 Der Text der Fußnote.</p></aside>'
+    return f"""<?xml version="1.0" encoding="utf-8"?>
+<html xmlns="http://www.w3.org/1999/xhtml" xmlns:epub="http://www.idpf.org/2007/ops">
+<head><title>Kapitel {n}</title></head>
+<body><h1>Kapitel {n}</h1>{''.join(paras)}{notes}</body></html>"""
+
+
+def main(path):
+    chapters = range(1, 13)
+    manifest = ''.join(f'<item id="c{n}" href="ch{n}.xhtml" media-type="application/xhtml+xml"/>' for n in chapters)
+    spine = ''.join(f'<itemref idref="c{n}"/>' for n in chapters)
+    nav_items = ''.join(f'<li><a href="ch{n}.xhtml">Kapitel {n}</a></li>' for n in chapters)
+    opf = f"""<?xml version="1.0" encoding="utf-8"?>
+<package xmlns="http://www.idpf.org/2007/opf" version="3.0" unique-identifier="id">
+  <metadata xmlns:dc="http://purl.org/dc/elements/1.1/">
+    <dc:identifier id="id">diora-e2e-book</dc:identifier>
+    <dc:title>E2E Testbuch</dc:title><dc:creator>diora</dc:creator><dc:language>de</dc:language>
+  </metadata>
+  <manifest><item id="nav" href="nav.xhtml" media-type="application/xhtml+xml" properties="nav"/>{manifest}</manifest>
+  <spine>{spine}</spine>
+</package>"""
+    nav = f"""<?xml version="1.0" encoding="utf-8"?>
+<html xmlns="http://www.w3.org/1999/xhtml" xmlns:epub="http://www.idpf.org/2007/ops">
+<head><title>Inhalt</title></head><body><nav epub:type="toc"><ol>{nav_items}</ol></nav></body></html>"""
+    with zipfile.ZipFile(path, 'w') as z:
+        z.writestr(zipfile.ZipInfo('mimetype'), 'application/epub+zip', compress_type=zipfile.ZIP_STORED)
+        z.writestr('META-INF/container.xml', CONTAINER, compress_type=zipfile.ZIP_DEFLATED)
+        z.writestr('OEBPS/content.opf', opf, compress_type=zipfile.ZIP_DEFLATED)
+        z.writestr('OEBPS/nav.xhtml', nav, compress_type=zipfile.ZIP_DEFLATED)
+        for n in chapters:
+            z.writestr(f'OEBPS/ch{n}.xhtml', chapter(n), compress_type=zipfile.ZIP_DEFLATED)
+
+
+if __name__ == '__main__':
+    main(sys.argv[1])
diff --git a/e2e/reader.spec.js b/e2e/reader.spec.js
new file mode 100644
index 0000000..9ea0654
--- /dev/null
+++ b/e2e/reader.spec.js
@@ -0,0 +1,142 @@
+// The reader end to end: a book goes in (encrypted in the browser), opens, is
+// read, and its place, a highlight and the theme survive a reload. The safety
+// net for refactoring app.js (docs/refactoring.md). One book for the whole file:
+// the tests build on each other and run in order.
+const path = require('path');
+const { test, expect } = require('@playwright/test');
+
+test.use({ storageState: path.join(__dirname, '.auth', 'user.json') });
+test.describe.configure({ mode: 'serial' });
+
+const BOOK = path.join(__dirname, '.auth', 'book.epub');
+let bookId = null;
+
+// Errors in the page fail the test that caused them.
+let pageErrors = [];
+test.beforeEach(({ page }) => {
+  pageErrors = [];
+  page.on('pageerror', e => pageErrors.push(String(e)));
+});
+test.afterEach(() => {
+  expect(pageErrors, 'errors in the page').toEqual([]);
+});
+
+async function openTestBook(page) {
+  await page.goto('/#books');
+  // A new service worker taking over reloads the page once: let that settle first.
+  await page.waitForLoadState('networkidle');
+  await page.waitForFunction(() => typeof openBook === 'function' && typeof _lastBookListData !== 'undefined' && Array.isArray(_lastBookListData));
+  await page.evaluate(id => openBook(id), bookId);
+  await page.waitForFunction(() => document.querySelectorAll('#reader-content [data-epub-src]').length === 12
+    && typeof _readPos !== 'undefined' && _readPos && _readPos.ready);
+}
+
+async function serverBook(page) {
+  const books = await (await page.request.get('/books/')).json();
+  return books.find(b => b.id === bookId);
+}
+
+test('a book goes in and shows in the list', async ({ page }) => {
+  await page.goto('/#books');
+  await page.setInputFiles('#book-file-input', BOOK);
+  const found = await page.waitForFunction(() => {
+    const list = typeof _lastBookListData !== 'undefined' ? (_lastBookListData || []) : [];
+    const b = list.find(x => x.title === 'E2E Testbuch');
+    return b ? b.id : null;
+  }, null, { timeout: 20000 });
+  bookId = await found.jsonValue();
+  expect(bookId).toBeTruthy();
+});
+
+test('opening without reading sends no position', async ({ page }) => {
+  const posts = [];
+  page.on('request', r => { if (r.method() === 'POST' && /\/progress\/$/.test(r.url())) posts.push(r.url()); });
+  await openTestBook(page);
+  await page.waitForTimeout(3000);
+  await page.evaluate(() => saveReaderProgress());
+  await page.evaluate(() => closeReader());
+  await page.waitForTimeout(500);
+  expect(posts).toEqual([]);
+});
+
+test('reading moves the position, and it comes back after a reload', async ({ page }) => {
+  await openTestBook(page);
+  const content = page.locator('#reader-content');
+  await content.hover();
+  for (let i = 0; i < 12; i++) await page.mouse.wheel(0, 600);
+  const saved = page.waitForRequest(r => r.method() === 'POST' && /\/progress\/$/.test(r.url()));
+  const body = JSON.parse((await saved).postData());
+  expect(body.device).toBe('Browser');
+  expect(body.read_at).toBeGreaterThan(1e12);
+  const anchor = await page.evaluate(() => getPositionAnchor(document.getElementById('reader-content')));
+  await page.waitForTimeout(500);
+  const b = await serverBook(page);
+  expect(b.position_anchor).not.toBe('');
+  expect(b.device).toBe('Browser');
+
+  await page.reload();
+  await openTestBook(page);
+  const restored = await page.evaluate(() => getPositionAnchor(document.getElementById('reader-content')));
+  const block = a => Number(a.split(':')[0]);
+  expect(Math.abs(block(restored) - block(anchor))).toBeLessThanOrEqual(1);
+});
+
+test('a footnote shows its text', async ({ page }) => {
+  await openTestBook(page);
+  const ref = page.locator('#reader-content [data-footnote-ref]').first();
+  await ref.scrollIntoViewIfNeeded();
+  await ref.click();
+  await expect(page.locator('.footnote-popover')).toContainText('Der Text der Fußnote');
+});
+
+test('a highlight is kept, and shows in the margin', async ({ page }) => {
+  await openTestBook(page);
+  // Marker mode on (the ✎ button), select the start of a paragraph on screen
+  // the way a mouse would, then let go.
+  await page.evaluate(() => {
+    if (!markerModeActive) toggleMarkerMode();
+    const p = document.querySelector('#reader-content #c1p2') || document.querySelector('#reader-content p');
+    p.scrollIntoView({ block: 'center' });
+    const text = p.firstChild;
+    const range = document.createRange();
+    range.setStart(text, 0);
+    range.setEnd(text, 20);
+    const sel = window.getSelection();
+    sel.removeAllRanges();
+    sel.addRange(range);
+    const r = range.getBoundingClientRect();
+    p.dispatchEvent(new MouseEvent('mouseup', { bubbles: true, clientX: r.right, clientY: r.bottom }));
+  });
+  const saved = page.waitForRequest(r => r.method() === 'POST' && /\/highlights\/$/.test(r.url()), { timeout: 15000 });
+  await page.locator('.hl-color-btn[data-hl-color="yellow"]').click();
+  await page.evaluate(() => saveHighlights && saveHighlights());
+  await saved;
+
+  await page.reload();
+  await openTestBook(page);
+  await page.waitForFunction(() => document.querySelectorAll('#reader-content mark.epub-highlight, #reader-content .epub-highlight').length > 0);
+  // The margin remembers being open; open it if it isn't.
+  await page.evaluate(() => { if (!readerAnnotationsMarginOpen) toggleAnnotationsMargin(); });
+  await expect(page.locator('#reader-margin-markers .margin-note')).toHaveCount(1);
+});
+
+test('the theme switch turns the page and the reader light, and stays', async ({ page }) => {
+  await page.goto('/');
+  await expect(page.locator('html')).toHaveAttribute('data-theme', 'dark');
+  await page.locator('#theme-toggle').click();
+  await expect(page.locator('html')).toHaveAttribute('data-theme', 'light');
+  await page.reload();
+  await expect(page.locator('html')).toHaveAttribute('data-theme', 'light');
+  await openTestBook(page);
+  await expect(page.locator('#reader-overlay')).toHaveClass(/reader-theme-bright/);
+  await page.evaluate(() => { closeReader(); __dioraSetTheme('dark'); });
+  await expect(page.locator('html')).toHaveAttribute('data-theme', 'dark');
+});
+
+test('every tab opens without errors', async ({ page }) => {
+  await page.goto('/');
+  for (const tab of ['home', 'radio', 'podcasts', 'books', 'feeds', 'focus', 'ambient']) {
+    await page.evaluate(t => showTab(t), tab);
+    await expect(page.locator(`#tab-${tab}`)).toBeVisible();
+  }
+});
diff --git a/playwright.config.js b/playwright.config.js
index 391972b..2c6bd11 100644
--- a/playwright.config.js
+++ b/playwright.config.js
@@ -6,7 +6,7 @@
 // migrated and seeded with a test user.
 const path = require('path');
 const { defineConfig, devices } = require('@playwright/test');
-const { BASE_URL, SERVER_ENV } = require('./e2e/env');
+const { BASE_URL, PORT, PYTHON, SERVER_ENV } = require('./e2e/env');
 
 module.exports = defineConfig({
   testDir: './e2e',
@@ -27,9 +27,10 @@ module.exports = defineConfig({
   ],
   globalSetup: require.resolve('./e2e/global-setup.js'),
   webServer: {
-    command: 'python3 manage.py runserver 127.0.0.1:8000 --noreload',
+    command: `${PYTHON} manage.py runserver 127.0.0.1:${PORT} --noreload`,
     url: BASE_URL + '/accounts/login/',
-    reuseExistingServer: !process.env.CI,
+    // Never reuse: a server already on the port has someone's real database behind it.
+    reuseExistingServer: false,
     timeout: 30_000,
     cwd: path.join(__dirname),
     env: SERVER_ENV,