chore: 全面规范化代码，遵循 CLAUDE.md 编码规范

- C# 文件命名改为 snake_case（28 个文件重命名）
- C# 类转换为主构造函数（8 个类）
- 空 catch 添加异常类型（2 处）
- 新建 GlobalUsings.cs（Health.Infrastructure、Health.WebApi）
- Flutter 移除 go_router，改用 Riverpod 路由栈
- Flutter 移除 flutter_secure_storage，改用 sqflite 持久化
- 修复 Flutter 构建路径（Flutter SDK 迁至 D 盘）
- 后端端口改为 0.0.0.0:5000，支持局域网访问

health_app/lib/core/local_database.dart

@@ -0,0 +1,58 @@
+import 'package:sqflite/sqflite.dart';
+import 'package:path/path.dart';
+
+/// SQLite 本地数据库——存储 token 等关键信息
+class LocalDatabase {
+  static LocalDatabase? _instance;
+  Database? _db;
+
+  LocalDatabase._();
+
+  static LocalDatabase get instance => _instance ??= LocalDatabase._();
+
+  Future<Database> get database async {
+    _db ??= await _initDb();
+    return _db!;
+  }
+
+  Future<Database> _initDb() async {
+    final dbPath = await getDatabasesPath();
+    final path = join(dbPath, 'health_app.db');
+    return openDatabase(
+      path,
+      version: 1,
+      onCreate: (db, version) async {
+        await db.execute(
+          'CREATE TABLE kv_store (key TEXT PRIMARY KEY, value TEXT)',
+        );
+      },
+    );
+  }
+
+  Future<void> write(String key, String value) async {
+    final db = await database;
+    await db.insert(
+      'kv_store',
+      {'key': key, 'value': value},
+      conflictAlgorithm: ConflictAlgorithm.replace,
+    );
+  }
+
+  Future<String?> read(String key) async {
+    final db = await database;
+    final result =
+        await db.query('kv_store', where: 'key = ?', whereArgs: [key]);
+    if (result.isEmpty) return null;
+    return result.first['value'] as String?;
+  }
+
+  Future<void> delete(String key) async {
+    final db = await database;
+    await db.delete('kv_store', where: 'key = ?', whereArgs: [key]);
+  }
+
+  Future<void> deleteAll() async {
+    final db = await database;
+    await db.delete('kv_store');
+  }
+}