Add IndexedDB book cache and fix PDF position on mobile

- Cache encrypted book data in IndexedDB (cache-first on open)
- Evict books not read for 4 weeks on book list load
- Fix PDF paginated mode not activating on book open (mobile)
- enterPdfPaginatedMode() now called after position restore in openBook()

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

static/js/app.js

@@ -2617,6 +2617,65 @@ function restoreFromAnchor(contentEl, anchor) {
   return true;
 }
 
+// ---------------------------------------------------------------------------
+// Book cache — IndexedDB, evict after 4 weeks of inactivity
+// ---------------------------------------------------------------------------
+const _BOOK_CACHE_STORE = 'books';
+const _BOOK_CACHE_EVICT_MS = 4 * 7 * 24 * 60 * 60 * 1000;
+
+function _openBookCacheDb() {
+  return new Promise((resolve, reject) => {
+    const req = indexedDB.open('diora_book_cache', 1);
+    req.onupgradeneeded = e => e.target.result.createObjectStore(_BOOK_CACHE_STORE, {keyPath: 'id'});
+    req.onsuccess  = e => resolve(e.target.result);
+    req.onerror    = ()  => reject();
+  });
+}
+
+async function _getCachedBook(bookId) {
+  try {
+    const db = await _openBookCacheDb();
+    return await new Promise((resolve) => {
+      const req = db.transaction(_BOOK_CACHE_STORE).objectStore(_BOOK_CACHE_STORE).get(bookId);
+      req.onsuccess = e => resolve(e.target.result || null);
+      req.onerror   = ()  => resolve(null);
+    });
+  } catch { return null; }
+}
+
+async function _setCachedBook(bookId, data_ct, data_iv) {
+  try {
+    const db = await _openBookCacheDb();
+    await new Promise((resolve) => {
+      const tx = db.transaction(_BOOK_CACHE_STORE, 'readwrite');
+      tx.objectStore(_BOOK_CACHE_STORE).put({id: bookId, data_ct, data_iv, cached_at: Date.now()});
+      tx.oncomplete = resolve;
+      tx.onerror    = resolve;
+    });
+  } catch {}
+}
+
+async function _evictBookCache(bookList) {
+  try {
+    const db = await _openBookCacheDb();
+    const now = Date.now();
+    const metaById = Object.fromEntries(bookList.map(b => [b.id, b]));
+    await new Promise((resolve) => {
+      const tx = db.transaction(_BOOK_CACHE_STORE, 'readwrite');
+      const store = tx.objectStore(_BOOK_CACHE_STORE);
+      store.openCursor().onsuccess = e => {
+        const cursor = e.target.result;
+        if (!cursor) { resolve(); return; }
+        const meta = metaById[cursor.value.id];
+        const lastRead = meta?.last_read ? new Date(meta.last_read).getTime() : 0;
+        if (!meta || (now - lastRead) > _BOOK_CACHE_EVICT_MS) cursor.delete();
+        cursor.continue();
+      };
+      tx.onerror = resolve;
+    });
+  } catch {}
+}
+
 // Reader settings
 let readerSettings = { fontSize: 16, lineHeight: 1.8, maxWidth: 65, theme: 'dark',
                         pdfZoom: 100, pdfInverted: false, pdfPaginated: false };
@@ -2668,6 +2727,7 @@ async function loadBookList() {
       return;
     }
     const books = await res.json();
+    _evictBookCache(books); // fire-and-forget
     if (!Array.isArray(books)) {
       listEl.innerHTML = `<p class="muted">Unexpected response from server (not an array).</p>`;
       return;
@@ -2996,8 +3056,15 @@ async function openBook(bookId) {
   try {
     loadReaderSettings();
     const key = await getOrCreateEncKey();
+    let data_ct, data_iv;
+    const cached = await _getCachedBook(bookId);
+    if (cached) {
+      ({data_ct, data_iv} = cached);
+    } else {
       const res = await fetch(`/books/${bookId}/data/`);
-    const {data_ct, data_iv} = await res.json();
+      ({data_ct, data_iv} = await res.json());
+      _setCachedBook(bookId, data_ct, data_iv); // fire-and-forget
+    }
     const plain = await decryptBytes(key, data_iv, data_ct);
 
     // Revoke any previous image blob URLs
@@ -3098,8 +3165,10 @@ async function openBook(bookId) {
       const fraction = bookData ? (bookData.scroll_fraction || 0) : 0;
       const anchor = bookData ? (bookData.position_anchor || '') : '';
       _currentPositionAnchor = anchor;
-      if (isPdf && readerSettings.pdfPaginated && pdfTotalPages > 1) {
-        if (fraction > 0) pdfCurrentPage = Math.max(1, Math.round(fraction * (pdfTotalPages - 1)) + 1);
+      if (isPdf && readerSettings.pdfPaginated) {
+        if (fraction > 0 && pdfTotalPages > 1)
+          pdfCurrentPage = Math.max(1, Math.round(fraction * (pdfTotalPages - 1)) + 1);
+        enterPdfPaginatedMode();
       } else if (!isPdf) {
         // Wait for all images so scrollHeight is final
         const imgs = Array.from(contentEl.querySelectorAll('img'));