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/ru.json

@@ -48,6 +48,7 @@
   "player_play": "Воспроизвести",
   "player_pause": "Пауза",
   "player_speed_label": "Скорость {speed}x",
+  "player_seek_label": "Прогресс главы",
   "player_change_speed": "Изменить скорость",
   "player_auto_next_on": "Автопереход вкл.",
   "player_auto_next_off": "Автопереход выкл.",
@@ -99,6 +100,7 @@
   "catalogue_results_count": "{n} результатов",
 
   "book_detail_page_title": "{title} — libnovel",
+  "book_detail_signin_to_save": "Войдите, чтобы сохранить",
   "book_detail_add_to_library": "В библиотеку",
   "book_detail_remove_from_library": "Удалить из библиотеки",
   "book_detail_read_now": "Читать",
@@ -123,6 +125,8 @@
   "reader_page_title": "{title} — Гл.{n} — libnovel",
   "reader_play_narration": "Воспроизвести озвучку",
   "reader_generating_audio": "Генерация аудио…",
+  "reader_signin_for_audio": "Доступна аудионарративация",
+  "reader_signin_audio_desc": "Войдите, чтобы слушать эту главу в озвучке ИИ.",
   "reader_audio_error": "Ошибка генерации аудио.",
   "reader_prev_chapter": "Предыдущая глава",
   "reader_next_chapter": "Следующая глава",