fix: offline mode migration bug for v1.5.0 → v1.6.2 updates

- Fixes offline mode incorrectly enabled after updating from v1.5.0
- Users with existing server configuration no longer appear as offline
- Root cause: KEY_OFFLINE_MODE didn't exist in v1.5.0
- MainViewModel/NoteEditorViewModel used hardcoded default 'true'
- Fix: Migration in SimpleNotesApplication.onCreate() detects server config
- Version bumped to v1.6.2 (versionCode 16)
- F-Droid changelogs added

Tested: v1.5.0 → v1.6.2 update successful
Migration log: hasServer=true → offlineMode=false ✓

fastlane/metadata/android/en-US/changelogs/16.txt

@@ -0,0 +1,2 @@
+• Fixes offline mode issue after updating from v1.5.0
+• Users with configured servers are no longer incorrectly shown as offline