feat: Phase 1 foundation - core infrastructure and navigation

Clean Architecture structure:
- core/config - Environment configuration
- core/theme - Material 3 theming with FlexColorScheme
- core/error - Typed exception hierarchy
- core/api - Dio HTTP clients with interceptors
- core/auth - Authentication state management
- routing - go_router with shell navigation
- shared/layouts - Adaptive scaffold

Dependencies added:
- flutter_riverpod, riverpod_annotation, riverpod_generator
- freezed, freezed_annotation, json_serializable
- dio, go_router, shared_preferences
- flex_color_scheme, flutter_adaptive_scaffold

Features:
- Responsive navigation (rail on desktop, bottom on mobile)
- Dashboard placeholder with welcome card
- Theme switching infrastructure
- API client ready for Core API and Tatlock API

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

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

lib/core/auth/auth_state.dart

@@ -0,0 +1,26 @@
+import 'package:freezed_annotation/freezed_annotation.dart';
+
+part 'auth_state.freezed.dart';
+
+/// Authentication state.
+@freezed
+class AuthState with _$AuthState {
+  const factory AuthState({
+    @Default(false) bool isAuthenticated,
+    String? accessToken,
+    String? refreshToken,
+    DateTime? expiresAt,
+    String? userId,
+    String? userName,
+    String? userEmail,
+  }) = _AuthState;
+
+  const AuthState._();
+
+  /// Check if token is expired or will expire soon.
+  bool get isTokenExpired {
+    if (expiresAt == null) return true;
+    // Consider expired if less than 1 minute remaining
+    return DateTime.now().isAfter(expiresAt!.subtract(const Duration(minutes: 1)));
+  }
+}