Support Google Services based sign-in config

lib/google_calendar_page.dart

@@ -30,7 +30,7 @@ class _AccountsPageState extends State<AccountsPage> {
   final googleSignIn = GoogleSignIn.instance;
   List<GoogleCalendarInfo> googleCalendars = [];
   GoogleSignInAccount? signedInAccount;
-  String runtimeServerClientId = googleCalendarServerClientId;
+  String? runtimeServerClientId;
   bool initialized = false;
   bool loadingCalendars = false;
   bool syncing = false;
@@ -53,16 +53,7 @@ class _AccountsPageState extends State<AccountsPage> {
 
     final configuredClientId = await _loadGoogleServerClientId();
     if (!mounted) return;
-    if (configuredClientId == null || configuredClientId.trim().isEmpty) {
-      setState(() {
-        loadingCalendars = false;
-        statusMessage =
-            'A Google napt\u00e1rkapcsolat m\u00e9g nincs bek\u00f6tve. A csal\u00e1dtagoknak nem kell client ID-t megadniuk; a JARVIS admin fel\u00fcleten kell egyszer be\u00e1ll\u00edtani a Google Calendar OAuth azonos\u00edt\u00f3t.';
-      });
-      return;
-    }
-
-    runtimeServerClientId = configuredClientId.trim();
+    runtimeServerClientId = configuredClientId?.trim();
     await _initializeGoogle();
   }
 
@@ -121,7 +112,10 @@ class _AccountsPageState extends State<AccountsPage> {
       if (!mounted) return;
       setState(() {
         loadingCalendars = false;
-        statusMessage = 'Google inicializ\u00e1l\u00e1s nem siker\u00fclt: $error';
+        statusMessage = _friendlyGoogleError(
+          'Google inicializ\u00e1l\u00e1s nem siker\u00fclt',
+          error,
+        );
       });
     }
   }
@@ -163,17 +157,30 @@ class _AccountsPageState extends State<AccountsPage> {
       if (!mounted) return;
       setState(() {
         loadingCalendars = false;
-        statusMessage = 'Google bejelentkez\u00e9s nem siker\u00fclt: $error';
+        statusMessage = _friendlyGoogleError(
+          'Google bejelentkez\u00e9s nem siker\u00fclt',
+          error,
+        );
       });
     }
   }
 
+  String _friendlyGoogleError(String prefix, Object error) {
+    final raw = error.toString();
+    if (raw.contains('serverClientId must be provided') ||
+        raw.contains('CredentialManager requires a serverClientId')) {
+      return '$prefix: hi\u00e1nyzik az app Google konfigur\u00e1ci\u00f3ja. '
+          'A bel\u00e9p\u00e9si ablakos m\u0171k\u00f6d\u00e9shez tedd be az android/app/google-services.json f\u00e1jlt, '
+          'amely tartalmaz Web application OAuth client ID-t, majd buildeld \u00fajra az APK-t.';
+    }
+    return '$prefix: $raw';
+  }
+
   Future<void> _ensureGoogleInitialized() async {
-    final clientId = runtimeServerClientId.trim();
-    if (clientId.isEmpty) {
-      throw Exception(
-        'A Google napt\u00e1rkapcsolat m\u00e9g nincs bek\u00f6tve.',
-      );
+    final clientId = runtimeServerClientId;
+    if (clientId == null || clientId.isEmpty) {
+      await googleSignIn.initialize();
+      return;
     }
 
     await googleSignIn.initialize(