feat(auth): implement dual-flow authentication (web + mobile)

Add complete authentication system supporting both web (NPM forward auth)
and mobile (OIDC) authentication flows.

Web flow:
- Check /auth/me on startup to detect NPM forward auth session
- Cookies handled by proxy, no Bearer tokens needed

Mobile flow:
- flutter_appauth for OIDC Authorization Code + PKCE
- POST /auth/sync to get user profile and roles
- Token storage in SharedPreferences

Shared:
- Permission system with Domain/Action enums and Role class
- PermissionGate and AdminGate widgets for UI permission checks
- Route guards redirecting unauthenticated users to login
- Login page with platform-specific messaging

Platform config:
- iOS: CFBundleURLTypes for net.schweitz.tatlock://
- Android: appAuthRedirectScheme, minSdk 23

Docs:
- Added Freezed 3.x sealed class documentation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/ARCHITECTURE.md

@@ -142,7 +142,7 @@ part 'container_model.freezed.dart';
 part 'container_model.g.dart';
 
 @freezed
-class ContainerModel with _$ContainerModel {
+sealed class ContainerModel with _$ContainerModel {
   const factory ContainerModel({
     required String id,
     required String name,
@@ -464,6 +464,34 @@ Generated files:
 - `*.freezed.dart` - Immutable classes
 - `*.g.dart` - JSON serialization, Riverpod providers
 
+### Freezed 3.x: Required `sealed class`
+
+**Freezed 3.x requires the `sealed` keyword** on all classes with generated mixins. Without it, the generated code will fail to compile with errors about missing concrete implementations.
+
+```dart
+// ✅ Correct - Freezed 3.x
+@freezed
+sealed class UserModel with _$UserModel {
+  const factory UserModel({
+    required String id,
+    required String name,
+  }) = _UserModel;
+
+  factory UserModel.fromJson(Map<String, dynamic> json) =>
+      _$UserModelFromJson(json);
+}
+
+// ❌ Wrong - will fail to compile
+@freezed
+class UserModel with _$UserModel {  // Missing `sealed`
+  const factory UserModel({...}) = _UserModel;
+}
+```
+
+The `sealed` keyword was introduced in Dart 3.0 and allows the generated mixin `_$UserModel` to have abstract members that are implemented by the private `_UserModel` class.
+
+**Always use `sealed class` with `@freezed`** - this applies to all models, entities, and state classes using Freezed.
+
 ## Import Rules
 
 1. Never import from `data/` in `domain/`