feat: admin controls on book detail page (cover/desc/chapter-names/audio TTS)

Add 5 admin sections to /books/[slug] for admins:
- Book cover generation (CF AI image-gen with preview + save)
- Chapter cover generation (chapter number input + preview)
- Description regeneration (preview + apply/discard)
- Chapter names generation (preview table + apply/discard)
- Audio TTS bulk enqueue (voice selector, chapter range, cancel)

Also adds /api/admin/audio/bulk and /api/admin/audio/cancel-bulk proxy routes,
and all i18n keys across en/fr/id/pt/ru.

ui/src/lib/paraglide/messages.js

@@ -1,4 +1,2 @@
 /* eslint-disable */
-export * from './messages/_index.js'
-// enabling auto-import by exposing all messages as m
-export * as m from './messages/_index.js'
+export * from './messages/_index.js'