Initialize Firebase at startup so iOS APNs token is captured

Firebase was only initialized lazily when the user toggled push on. On iOS the
firebase_messaging plugin installs its APNs swizzling at app launch, but the
delivered device token can only be forwarded to FCM if a FirebaseApp is already
configured at that point. With lazy init there was none, so getAPNSToken() never
resolved and enabling push failed with "Could not enable push".

Add initFirebaseForPush() (mobile-only, idempotent) and call it from main() at
launch. Extract a shared pushSupportedOnThisPlatform getter so isSupported and
the initializer stay in sync; _ensureFirebase() now delegates to it as a safety
net.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/lib/main.dart

@@ -5,6 +5,7 @@ import 'package:flutter/material.dart';
 import 'package:frontend/theme/catppuccin_mocha_theme.dart';
 import 'package:frontend/utils/local_network_permission.dart';
 import 'package:frontend/utils/pref_utils.dart';
+import 'package:frontend/utils/push_service.dart';
 import 'package:provider/provider.dart';
 import 'navigation.dart';
 import 'theme/gruvbox_theme.dart';
@@ -31,6 +32,15 @@ void main() {
         );
       }
 
+      // Initialize Firebase at launch so the firebase_messaging plugin's iOS
+      // APNs swizzling has a configured app to forward the device token to;
+      // without this getAPNSToken() never resolves and enabling push fails.
+      try {
+        await initFirebaseForPush();
+      } catch (e, stack) {
+        debugPrint('Firebase init for push failed, continuing: $e\n$stack');
+      }
+
       // Settle iOS's local-network permission now, at launch, so it isn't
       // still being decided when the user first taps "Test" in settings.
       unawaited(requestLocalNetworkPermission());