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/src/routes/+layout.server.ts

@@ -4,11 +4,15 @@ import { getSettings } from '$lib/server/pocketbase';
 import { log } from '$lib/server/logger';
 
 // Routes that are accessible without being logged in
-const PUBLIC_ROUTES = new Set(['/login', '/disclaimer', '/privacy', '/dmca', '/terms']);
+const PUBLIC_ROUTES = new Set(['/login', '/disclaimer', '/privacy', '/dmca', '/terms', '/catalogue']);
 
 export const load: LayoutServerLoad = async ({ locals, url, cookies }) => {
-	// Allow /auth/* (OAuth initiation + callbacks) without login
-	const isPublic = PUBLIC_ROUTES.has(url.pathname) || url.pathname.startsWith('/auth/');
+	// Allow public routes, /auth/*, and all book-browsing URLs (/books/[slug] and deeper)
+	// Note: /books (the personal library) is intentionally NOT public
+	const isPublic =
+		PUBLIC_ROUTES.has(url.pathname) ||
+		url.pathname.startsWith('/auth/') ||
+		url.pathname.startsWith('/books/');
 	if (!isPublic && !locals.user) {
 		redirect(302, `/login`);
 	}