feat: add anonymous mode toggle to hide reading progress and tracking

Adds a toggleable anonymous mode (eye icon in header) that:
- Stops syncing read progress to the server while reading
- Hides mark as read/unread buttons on book covers and lists
- Hides reading status badges on series and books
- Hides progress bars on series and book covers
- Hides "continue reading" and "continue series" sections on home
- Persists the setting server-side in user preferences (anonymousMode)

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

src/i18n/messages/fr/common.json

@@ -450,6 +450,9 @@
   "header": {
     "toggleSidebar": "Afficher/masquer le menu latéral",
     "toggleTheme": "Changer le thème",
+    "anonymousMode": "Mode anonyme",
+    "anonymousModeOn": "Mode anonyme activé",
+    "anonymousModeOff": "Mode anonyme désactivé",
     "search": {
       "placeholder": "Rechercher séries et tomes...",
       "empty": "Aucun résultat",