docs(user): align sync and user guides with current device support

- sync-guide: only Web and KOReader are fully supported; move Kobo to
  coming soon, drop fake Q2-Q4 2026 release dates for mobile/Kindle/
  Remarkable, and describe the plugin + server-approval registration
  flow instead of QR-code/URL approval
- sync-guide: remove cellular/mobile-app advice from battery and
  best-practice sections, correct the Calibre compatibility FAQ, update
  the changelog to reflect shipped vs. pending sync features, and fix
  the license header (GPL-3.0, not MIT)
- user-guide: lead device setup with KOReader; mark the Kobo guide as
  coming soon
- calibre-integration: OPDS client list no longer implies native Kobo
  support
- auth overview: label the mobile-application token guidance as
  'coming later' since no mobile apps exist yet

docs/developer/api/authentication/overview.md

@@ -158,7 +158,7 @@ The frontend toast.js interceptor:
 - **Backend**: Automatically manages HTTP-only cookie
 - **Frontend**: Store tokens in localStorage for API calls
 
-### Mobile Applications
+### Mobile Applications (coming later)
 
 - Store access token in secure storage (Keychain/Keystore)
 - Store refresh token in secure storage