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/error/app_exception.dart

@@ -0,0 +1,53 @@
+/// Base exception for all application errors.
+sealed class AppException implements Exception {
+  const AppException({required this.message, this.cause});
+
+  final String message;
+  final Object? cause;
+
+  @override
+  String toString() => 'AppException: $message';
+}
+
+/// Network-related errors (connection, timeout, etc.)
+class NetworkException extends AppException {
+  const NetworkException({required super.message, super.cause});
+}
+
+/// API errors with HTTP status codes.
+class ApiException extends AppException {
+  const ApiException({
+    required super.message,
+    required this.statusCode,
+    this.code,
+    super.cause,
+  });
+
+  final int statusCode;
+  final String? code;
+
+  bool get isUnauthorized => statusCode == 401;
+  bool get isForbidden => statusCode == 403;
+  bool get isNotFound => statusCode == 404;
+  bool get isServerError => statusCode >= 500;
+}
+
+/// Authentication-related errors.
+class AuthException extends AppException {
+  const AuthException({required super.message, super.cause});
+}
+
+/// Cache/storage errors.
+class CacheException extends AppException {
+  const CacheException({required super.message, super.cause});
+}
+
+/// Validation errors.
+class ValidationException extends AppException {
+  const ValidationException({
+    required super.message,
+    this.fieldErrors = const {},
+  });
+
+  final Map<String, String> fieldErrors;
+}