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/config/app_config.dart

@@ -0,0 +1,40 @@
+/// Application configuration from compile-time environment variables.
+///
+/// Set via: flutter build --dart-define=API_URL=https://...
+class AppConfig {
+  AppConfig._();
+
+  /// Core API base URL
+  static const coreApiUrl = String.fromEnvironment(
+    'CORE_API_URL',
+    defaultValue: 'https://api.schweitz.net',
+  );
+
+  /// Tatlock API base URL
+  static const tatlockApiUrl = String.fromEnvironment(
+    'TATLOCK_API_URL',
+    defaultValue: 'https://tatlock.schweitz.net',
+  );
+
+  /// Authentik OIDC discovery URL
+  static const authDiscoveryUrl = String.fromEnvironment(
+    'AUTH_DISCOVERY_URL',
+    defaultValue:
+        'https://auth.schweitz.net/application/o/tatlock-ui/.well-known/openid-configuration',
+  );
+
+  /// Authentik client ID
+  static const authClientId = String.fromEnvironment(
+    'AUTH_CLIENT_ID',
+    defaultValue: 'tatlock-ui',
+  );
+
+  /// Authentik redirect URI scheme
+  static const authRedirectScheme = String.fromEnvironment(
+    'AUTH_REDIRECT_SCHEME',
+    defaultValue: 'net.schweitz.tatlock',
+  );
+
+  /// Whether running in debug mode
+  static const isDebug = bool.fromEnvironment('DEBUG', defaultValue: false);
+}