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/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         -->