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/user/user-guide.md

@@ -6,18 +6,16 @@ Welcome to the Bookhoard user documentation. This section contains guides for us
 
 Learn how to configure your e-reader devices to sync with Bookhoard:
 
-- **[Kobo Setup Guide](devices/kobo-setup.md)** - Complete guide for Kobo e-readers
-  - Device registration
-  - Sync configuration
-  - OPDS wireless book delivery
-  - Troubleshooting
-
 - **[KOReader Setup Guide](devices/koreader-setup.md)** - Complete guide for KOReader
   - Installation on Kindle/Kobo/PocketBook
-  - Sync setup
+  - Plugin setup with server-side device approval
+  - Progress, bookmark, highlight, and note sync
   - OPDS catalog access
   - Troubleshooting
 
+- **[Kobo Setup Guide](devices/kobo-setup.md)** - Native Kobo sync (coming soon)
+  - In the meantime, KOReader works great on Kobo hardware
+
 ## 🔄 Sync Configuration
 
 - **[Universal Sync Guide](sync-guide.md)** - Understanding and using sync features