perf: 性能优化与代码质量改进

- 添加4个数据库索引提升查询性能
- 启用ESLint并配置React支持
- 修复ESLint错误（未使用的导入和状态）
- 简化Toast组件UI样式
- 移除重复文件(server.js.backup)
- 清理.gitignore中的.next目录

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server.js

@@ -66,6 +66,11 @@ db.exec(`
     created_at DATETIME DEFAULT CURRENT_TIMESTAMP,
     FOREIGN KEY (user_id) REFERENCES users(id)
   );
+
+  CREATE INDEX IF NOT EXISTS idx_records_user_month ON records(user_id, date);
+  CREATE INDEX IF NOT EXISTS idx_records_user_category ON records(user_id, category);
+  CREATE INDEX IF NOT EXISTS idx_recurring_user ON recurring_bills(user_id);
+  CREATE INDEX IF NOT EXISTS idx_categories_user ON categories(user_id);
 `);
 
 const VALID_RECORD_TYPES = ['income', 'expense'];
@@ -312,6 +317,9 @@ app.get('/api/records', authenticate, (req, res) => {
   }
 
   if (type && type !== 'all') {
+    if (!VALID_RECORD_TYPES.includes(type)) {
+      return res.status(400).json({ error: '类型参数错误' });
+    }
     query += ' AND type = ?';
     params.push(type);
   }
@@ -808,7 +816,7 @@ app.get('/_next/static/:path(*)', (req, res) => {
 
 app.get('*', (req, res) => {
   // 检查是否是 Next.js 预渲染的页面
-  let pagePath = req.path === '/' ? '/index.html' : req.path + '.html';
+  const pagePath = req.path === '/' ? '/index.html' : req.path + '.html';
   const nextPagePath = path.join(__dirname, 'frontend/.next/server/pages', pagePath);
 
   res.sendFile(nextPagePath, (err) => {