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>

test/widget_test.dart

@@ -1,12 +1,19 @@
+import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:flutter_test/flutter_test.dart';
 
-import 'package:tatlock_ui/main.dart';
+import 'package:tatlock_ui/app.dart';
 
 void main() {
-  testWidgets('App renders home page', (WidgetTester tester) async {
-    await tester.pumpWidget(const TatlockApp());
+  testWidgets('App renders dashboard', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const ProviderScope(
+        child: TatlockApp(),
+      ),
+    );
 
-    expect(find.text('Tatlock UI'), findsOneWidget);
-    expect(find.text('Welcome to Tatlock UI'), findsOneWidget);
+    // Wait for async operations
+    await tester.pumpAndSettle();
+
+    expect(find.text('Welcome to Tatlock'), findsOneWidget);
   });
 }