fix: Android 9 crash - Implement getForegroundInfo() for WorkManager Expedited Work (Issue #15)

This commit fixes the critical crash on Android 9 (API 28) that occurred when using
WorkManager Expedited Work for background sync operations.

## Root Cause
When setExpedited() is used in WorkManager, the CoroutineWorker must implement
getForegroundInfo() to return a ForegroundInfo object with a Foreground Service
notification. On Android 9-11, WorkManager calls this method, but the default
implementation throws: IllegalStateException: Not implemented

## Solution
- Implemented getForegroundInfo() in SyncWorker
- Returns ForegroundInfo with sync progress notification
- Android 10+: Sets FOREGROUND_SERVICE_TYPE_DATA_SYNC for proper service typing
- Added required Foreground Service permissions to AndroidManifest.xml

## Technical Changes
- SyncWorker.kt: Added getForegroundInfo() override
- NotificationHelper.kt: Added createSyncProgressNotification() factory method
- strings.xml: Added sync_in_progress UI strings (EN + DE)
- AndroidManifest.xml: Added FOREGROUND_SERVICE permissions
- Version updated to 1.7.1 (versionCode 18)

## Previously Fixed (in this release)
- Kernel-VPN compatibility (Wireguard interface detection)
- HTTP connection lifecycle optimization (SafeSardineWrapper)
- Stability improvements for sync sessions

## Testing
- Tested on Android 9 (API 28) - No crash on second app start
- Tested on Android 15 (API 35) - No regressions
- WiFi-connect sync working correctly
- Expedited work notifications display properly

Fixes #15
Thanks to @roughnecks for detailed bug report and testing!

android/app/src/main/res/values/strings.xml

@@ -438,6 +438,8 @@
     <!-- ============================= -->
     <string name="notification_channel_name">Notes Synchronization</string>
     <string name="notification_channel_desc">Notifications about sync status</string>
+    <string name="sync_in_progress">Syncing</string>
+    <string name="sync_in_progress_text">Syncing notes…</string>
     <string name="notification_sync_success_title">Sync successful</string>
     <string name="notification_sync_success_message">%d note(s) synchronized</string>
     <string name="notification_sync_failed_title">Sync failed</string>