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/static/robots.txt

@@ -1,3 +1,14 @@
-# allow crawling everything by default
 User-agent: *
-Disallow:
+Disallow: /books
+Disallow: /profile
+Disallow: /admin/
+Disallow: /api/
+Disallow: /auth/
+Allow: /books/
+Allow: /catalogue
+Allow: /login
+Allow: /disclaimer
+Allow: /privacy
+Allow: /dmca
+
+Sitemap: https://libnovel.cc/sitemap.xml