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/main.dart

@@ -1,7 +1,9 @@
 import 'dart:developer' as developer;
 
 import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
 
+import 'app.dart';
 import 'version.g.dart';
 
 void main() {
@@ -12,38 +14,9 @@ void main() {
     name: 'tatlock_ui',
   );
 
-  runApp(const TatlockApp());
-}
-
-class TatlockApp extends StatelessWidget {
-  const TatlockApp({super.key});
-
-  @override
-  Widget build(BuildContext context) {
-    return MaterialApp(
-      title: 'Tatlock UI',
-      theme: ThemeData(
-        colorScheme: ColorScheme.fromSeed(seedColor: Colors.teal),
-        useMaterial3: true,
-      ),
-      home: const HomePage(),
-    );
-  }
-}
-
-class HomePage extends StatelessWidget {
-  const HomePage({super.key});
-
-  @override
-  Widget build(BuildContext context) {
-    return Scaffold(
-      appBar: AppBar(
-        backgroundColor: Theme.of(context).colorScheme.inversePrimary,
-        title: const Text('Tatlock UI'),
-      ),
-      body: const Center(
-        child: Text('Welcome to Tatlock UI'),
-      ),
-    );
-  }
+  runApp(
+    const ProviderScope(
+      child: TatlockApp(),
+    ),
+  );
 }