Enhance BLE connection handling and improve USB connection messaging

- Wrapped BLE scan and connection methods in try-catch blocks to handle errors gracefully and provide debug output.
- Added retry logic for service discovery on web platforms after transient disconnections.
- Updated USB connection messages in multiple languages to reflect active support on Android and desktop platforms.
- Improved loading indicators for contacts screen to show a spinner during data loading.

lib/screens/contacts_screen.dart

@@ -384,8 +384,15 @@ class _ContactsScreenState extends State<ContactsScreen>
 
   Widget _buildContactsBody(BuildContext context, MeshCoreConnector connector) {
     final contacts = connector.contacts;
+    final shouldShowStartupSpinner =
+        contacts.isEmpty &&
+        _groups.isEmpty &&
+        connector.isConnected &&
+        (connector.isLoadingContacts ||
+            connector.isLoadingChannels ||
+            connector.selfPublicKey == null);
 
-    if (contacts.isEmpty && connector.isLoadingContacts && _groups.isEmpty) {
+    if (shouldShowStartupSpinner) {
       return const Center(child: CircularProgressIndicator());
     }