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/api/api_client.dart

@@ -0,0 +1,55 @@
+import 'package:dio/dio.dart';
+import 'package:riverpod_annotation/riverpod_annotation.dart';
+
+import '../config/app_config.dart';
+import 'api_interceptors.dart';
+
+part 'api_client.g.dart';
+
+/// Provides the Dio instance for Core API.
+@riverpod
+Dio coreApiClient(CoreApiClientRef ref) {
+  final dio = Dio(
+    BaseOptions(
+      baseUrl: AppConfig.coreApiUrl,
+      connectTimeout: const Duration(seconds: 10),
+      receiveTimeout: const Duration(seconds: 30),
+      headers: {
+        'Content-Type': 'application/json',
+        'Accept': 'application/json',
+      },
+    ),
+  );
+
+  dio.interceptors.addAll([
+    AuthInterceptor(ref),
+    LoggingInterceptor(),
+    ErrorInterceptor(),
+  ]);
+
+  return dio;
+}
+
+/// Provides the Dio instance for Tatlock API.
+@riverpod
+Dio tatlockApiClient(TatlockApiClientRef ref) {
+  final dio = Dio(
+    BaseOptions(
+      baseUrl: AppConfig.tatlockApiUrl,
+      connectTimeout: const Duration(seconds: 10),
+      receiveTimeout: const Duration(minutes: 5), // Longer for LLM responses
+      headers: {
+        'Content-Type': 'application/json',
+        'Accept': 'application/json',
+      },
+    ),
+  );
+
+  dio.interceptors.addAll([
+    AuthInterceptor(ref),
+    LoggingInterceptor(),
+    ErrorInterceptor(),
+  ]);
+
+  return dio;
+}