feat(1.5.0): add "Nothing to sync" banner and improve sync UX for v1.5.0

Code changes:
- Show "ℹ️ Nothing to sync" when no notes need syncing (banner instead of toast)
- Add i18n strings for EN/DE

Documentation:
- Improve auto-sync description: mention WiFi reconnect + multi-device sync
- Add 2 new screenshots: server settings & sync status banner

Assets:
- Add screenshots 5 & 6 (server settings & sync banner showcase)

android/app/src/main/java/dev/dettmer/simplenotes/ui/main/MainViewModel.kt

@@ -493,7 +493,12 @@ class MainViewModel(application: Application) : AndroidViewModel(application) {
                 }
                 
                 if (result.isSuccess) {
-                    SyncStateManager.markCompleted(getString(R.string.toast_sync_success, result.syncedCount))
+                    val bannerMessage = if (result.syncedCount > 0) {
+                        getString(R.string.toast_sync_success, result.syncedCount)
+                    } else {
+                        getString(R.string.snackbar_nothing_to_sync)
+                    }
+                    SyncStateManager.markCompleted(bannerMessage)
                     loadNotes()
                 } else {
                     SyncStateManager.markError(result.errorMessage)
@@ -566,7 +571,7 @@ class MainViewModel(application: Application) : AndroidViewModel(application) {
                     loadNotes()
                 } else if (result.isSuccess) {
                     Logger.d(TAG, "ℹ️ Auto-sync ($source): No changes")
-                    SyncStateManager.markCompleted()
+                    SyncStateManager.markCompleted(getString(R.string.snackbar_nothing_to_sync))
                 } else {
                     Logger.e(TAG, "❌ Auto-sync failed ($source): ${result.errorMessage}")
                     SyncStateManager.markError(result.errorMessage)

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

@@ -92,6 +92,7 @@
     <string name="snackbar_already_synced">Bereits synchronisiert</string>
     <string name="snackbar_server_unreachable">Server nicht erreichbar</string>
     <string name="snackbar_synced_count">✅ Gesynct: %d Notizen</string>
+    <string name="snackbar_nothing_to_sync">ℹ️ Nichts zu syncen</string>
     
     <!-- ============================= -->
     <!-- URL VALIDATION ERRORS         -->

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

@@ -93,6 +93,7 @@
     <string name="snackbar_already_synced">Already synced</string>
     <string name="snackbar_server_unreachable">Server not reachable</string>
     <string name="snackbar_synced_count">✅ Synced: %d notes</string>
+    <string name="snackbar_nothing_to_sync">ℹ️ Nothing to sync</string>
     
     <!-- ============================= -->
     <!-- URL VALIDATION ERRORS         -->