Accounts: fix password change key migration

The old key comparison was wrong — the localStorage key may have been
randomly generated rather than PBKDF2-derived. Fix:
- Add /accounts/check-password/ to validate the old password server-side
  before touching any keys
- Use the localStorage key directly as the old decryption key (it is
  always the correct source of truth, regardless of how it was generated)
- Derive the new key from the new password via PBKDF2

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

accounts/urls.py

@@ -13,5 +13,6 @@ urlpatterns = [
     path('background/upload/', views.upload_background, name='upload_background'),
     path('background/delete/', views.delete_background, name='delete_background'),
     path('focus-station/', views.save_focus_station, name='save_focus_station'),
+    path('check-password/', views.check_password, name='check_password'),
     path('change-password/', views.change_password, name='change_password'),
 ]

accounts/views.py

@@ -182,6 +182,19 @@ def save_focus_station(request):
     return JsonResponse({'ok': True})
 
 
+@login_required
+@csrf_exempt
+@require_http_methods(['POST'])
+def check_password(request):
+    try:
+        body = json.loads(request.body)
+    except (json.JSONDecodeError, ValueError):
+        return JsonResponse({'ok': False}, status=400)
+    pw = body.get('password', '')
+    user = authenticate(request, username=request.user.username, password=pw)
+    return JsonResponse({'ok': user is not None})
+
+
 @login_required
 @require_http_methods(['POST'])
 def change_password(request):

templates/accounts/settings.html

@@ -175,26 +175,31 @@ document.getElementById('pw-change-form')?.addEventListener('submit', async func
   const username = '{{ request.user.username }}';
   const storageKey = `diora_enc_key_${userId}`;
 
-  statusEl.textContent = 'Schlüssel ableiten…';
+  statusEl.textContent = 'Passwort prüfen…';
   try {
-    const oldKey = await _pbkdf2Key(oldPw, username);
-
-    // Verify old key matches localStorage
-    const stored = localStorage.getItem(storageKey);
-    if (stored) {
-      const storedBytes = _base64ToBytes(stored);
-      const derivedBytes = new Uint8Array(await crypto.subtle.exportKey('raw', oldKey));
-      if (!storedBytes.every((b, i) => b === derivedBytes[i])) {
-        statusEl.textContent = 'Fehler: Das alte Passwort stimmt nicht mit dem gespeicherten Schlüssel überein. Bitte altes Passwort prüfen.';
+    // Validate old password server-side before touching any keys
+    const checkResp = await fetch('/accounts/check-password/', {
+      method: 'POST',
+      headers: {'Content-Type': 'application/json', 'X-CSRFToken': getCsrfToken()},
+      body: JSON.stringify({password: oldPw}),
+    }).then(r => r.json());
+    if (!checkResp.ok) {
+      statusEl.textContent = 'Fehler: Altes Passwort ist falsch.';
       return;
     }
+
+    // Use the key currently in localStorage as the old key (source of truth for decryption)
+    const stored = localStorage.getItem(storageKey);
+    let oldKey;
+    if (stored) {
+      oldKey = await crypto.subtle.importKey('raw', _base64ToBytes(stored), {name: 'AES-GCM'}, false, ['decrypt']);
     }
 
     const newKey = await _pbkdf2Key(newPw, username);
 
-    // Re-encrypt all books
+    // Re-encrypt all books (only if we have a key to decrypt with)
     statusEl.textContent = 'Bücherliste laden…';
-    const books = await fetch('/books/').then(r => r.json());
+    const books = oldKey ? await fetch('/books/').then(r => r.json()) : [];
 
     for (let i = 0; i < books.length; i++) {
       const book = books[i];