feat: open catalogue + book pages to public (hybrid open model)

Reading content is now publicly accessible without login:
- /catalogue, /books/[slug], /books/[slug]/chapters, /books/[slug]/chapters/[n]
  are all public — no login redirect
- /books (personal library), /profile, /admin remain protected

Unauthenticated UX:
- Chapter page: "Audio narration available — Sign in to listen" banner
  replaces the audio player for logged-out visitors
- Book detail: bookmark icon links to /login instead of triggering
  the save action (both desktop and mobile CTAs)

SEO:
- robots.txt updated: Allow /books/ and /catalogue, Disallow /books (library)
- sitemap.xml now includes /catalogue as primary crawl entry point

i18n: added reader_signin_for_audio, reader_signin_audio_desc,
      book_detail_signin_to_save in all 5 languages

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

ui/messages/fr.json

@@ -48,6 +48,7 @@
   "player_play": "Lecture",
   "player_pause": "Pause",
   "player_speed_label": "Vitesse {speed}x",
+  "player_seek_label": "Progression du chapitre",
   "player_change_speed": "Changer la vitesse",
   "player_auto_next_on": "Suivant auto activé",
   "player_auto_next_off": "Suivant auto désactivé",
@@ -99,6 +100,7 @@
   "catalogue_results_count": "{n} résultats",
 
   "book_detail_page_title": "{title} — libnovel",
+  "book_detail_signin_to_save": "Connectez-vous pour sauvegarder",
   "book_detail_add_to_library": "Ajouter à la bibliothèque",
   "book_detail_remove_from_library": "Retirer de la bibliothèque",
   "book_detail_read_now": "Lire maintenant",
@@ -123,6 +125,8 @@
   "reader_page_title": "{title} — Ch.{n} — libnovel",
   "reader_play_narration": "Lire la narration",
   "reader_generating_audio": "Génération audio…",
+  "reader_signin_for_audio": "Narration audio disponible",
+  "reader_signin_audio_desc": "Connectez-vous pour écouter ce chapitre narré par l'IA.",
   "reader_audio_error": "Échec de la génération audio.",
   "reader_prev_chapter": "Chapitre précédent",
   "reader_next_chapter": "Chapitre suivant",