perf: 性能优化与代码质量改进

- 简化ToastProvider组件，移除未使用导入
- 简化Stats页面代码
- 添加React ESLint配置
- 添加数据库索引和API参数验证
- 清理重复文件server.js.backup
- 移除前端未使用的UI代码和复杂样式

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

frontend/lib/swr-config.js

@@ -1,5 +1,3 @@
-import { getRecords, getStats, getCategories, getRecurringBills } from './api';
-
 // 全局 SWR 配置
 export const swrConfig = {
   refreshInterval: 0, // 默认不自动刷新
@@ -11,11 +9,3 @@ export const swrConfig = {
     console.error('SWR Error:', error);
   },
 };
-
-// 数据获取器映射
-export const fetchers = {
-  records: (month, type) => getRecords(month, type),
-  stats: (month) => getStats(month),
-  categories: () => getCategories(),
-  recurring: () => getRecurringBills(),
-};

frontend/lib/utils.js

@@ -7,6 +7,13 @@ export const formatMonth = (date) =>
 
 export const formatMoney = (num) => (num || 0).toFixed(2);
 
+// Format date as 'YYYY-MM-DD'
+export const formatDate = (date) =>
+  `${date.getFullYear()}-${String(date.getMonth() + 1).padStart(2, '0')}-${String(date.getDate()).padStart(2, '0')}`;
+
+// Get current time string 'HH:MM'
+export const getCurrentTimeStr = () => new Date().toTimeString().slice(0, 5);
+
 // Parse date-time string 'YYYY-MM-DD HH:MM' into date and time parts
 export const parseDateTime = (dateTimeStr) => {
   if (!dateTimeStr) return { datePart: '', timePart: '00:00' };