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/app.dart

@@ -0,0 +1,33 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+import 'core/theme/app_theme.dart';
+import 'core/theme/theme_provider.dart';
+import 'routing/app_router.dart';
+
+/// Root application widget.
+class TatlockApp extends ConsumerWidget {
+  const TatlockApp({super.key});
+
+  @override
+  Widget build(BuildContext context, WidgetRef ref) {
+    final router = ref.watch(appRouterProvider);
+    final themeAsync = ref.watch(themeNotifierProvider);
+
+    // Get theme mode, defaulting to system while loading
+    final themeMode = switch (themeAsync) {
+      ThemeSetting.system => ThemeMode.system,
+      ThemeSetting.light => ThemeMode.light,
+      ThemeSetting.dark => ThemeMode.dark,
+    };
+
+    return MaterialApp.router(
+      title: 'Tatlock UI',
+      debugShowCheckedModeBanner: false,
+      theme: AppTheme.light,
+      darkTheme: AppTheme.dark,
+      themeMode: themeMode,
+      routerConfig: router,
+    );
+  }
+}