refactor: migrate models to freezed + json_serializable

lib/screens/profile/add_local_profile_screen.dart

@@ -64,9 +64,8 @@ class _AddLocalProfileScreenState extends State<AddLocalProfileScreen> with Cont
     setState(() => _saving = true);
 
     final registry = context.read<ProfileRegistry>();
-    final profile = Profile(
+    final profile = Profile.local(
       id: 'local-${const Uuid().v4()}',
-      kind: ProfileKind.local,
       displayName: name,
       pinHash: _pinHash,
       sortOrder: DateTime.now().millisecondsSinceEpoch,

lib/screens/profile/profile_detail_screen.dart

@@ -87,14 +87,18 @@ class _ProfileDetailScreenState extends State<ProfileDetailScreen> with Controll
       onMismatch: (ctx) => showErrorSnackBar(ctx, t.profiles.pinsDontMatch),
     );
     if (pin == null || !mounted) return;
-    final updated = _profile.copyWith(pinHash: computePinHash(pin));
+    final profile = _profile;
+    if (profile is! LocalProfile) return;
+    final updated = profile.copyWith(pinHash: computePinHash(pin));
     await context.read<ProfileRegistry>().upsert(updated);
     if (!mounted) return;
     setState(() => _profile = updated);
   }
 
   Future<void> _clearPin() async {
-    final updated = _profile.copyWith(clearPin: true);
+    final profile = _profile;
+    if (profile is! LocalProfile) return;
+    final updated = profile.copyWith(pinHash: null);
     await context.read<ProfileRegistry>().upsert(updated);
     if (!mounted) return;
     setState(() => _profile = updated);

lib/screens/settings/add_jellyfin_screen.dart

@@ -264,9 +264,8 @@ class _AddJellyfinScreenState extends State<AddJellyfinScreen> with AsyncFormSta
       hasProfiles: activeProvider.profiles.isNotEmpty,
     )) {
       final now = DateTime.now();
-      final profile = Profile(
+      final profile = Profile.local(
         id: 'local-${const Uuid().v4()}',
-        kind: ProfileKind.local,
         displayName: connection.userName.isNotEmpty ? connection.userName : connection.serverName,
         sortOrder: now.millisecondsSinceEpoch,
         createdAt: now,