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/pages/index.js

@@ -7,7 +7,7 @@ import RecordModal from '../components/RecordModal';
 import { useToast } from '../components/ToastProvider';
 import { useConfirm } from '../components/ConfirmProvider';
 import { useCategories } from '../hooks/useCategories';
-import { getMonthStr, formatMoney, formatMonth, parseDateTime, getDatePart } from '../lib/utils';
+import { getMonthStr, formatMoney, formatMonth, parseDateTime, getDatePart, formatDate, getCurrentTimeStr } from '../lib/utils';
 
 export default function Home() {
   const router = useRouter();
@@ -31,12 +31,13 @@ export default function Home() {
       showToast('分类加载失败', { tone: 'error' });
     }
   }, [categoriesError]);
+
   const [formData, setFormData] = useState({
     type: 'expense',
     amount: '',
     category: '',
-    date: new Date().toISOString().split('T')[0],
-    time: new Date().toTimeString().slice(0, 5),
+    date: formatDate(new Date()),
+    time: getCurrentTimeStr(),
     note: '',
   });
 
@@ -53,8 +54,8 @@ export default function Home() {
     type: 'expense',
     amount: '',
     category: '',
-    date: new Date().toISOString().split('T')[0],
-    time: new Date().toTimeString().slice(0, 5),
+    date: formatDate(new Date()),
+    time: getCurrentTimeStr(),
     note: '',
   });
 
@@ -206,12 +207,16 @@ export default function Home() {
     [groupedRecords]
   );
 
-  const getDayExpense = (date) => {
-    const dayRecords = groupedRecords[date] || [];
-    return dayRecords
-      .filter((record) => record.type === 'expense')
-      .reduce((sum, record) => sum + record.amount, 0);
-  };
+  const dayExpenses = useMemo(() => {
+    const expenses = {};
+    for (const date of Object.keys(groupedRecords)) {
+      const dayRecords = groupedRecords[date] || [];
+      expenses[date] = dayRecords
+        .filter((record) => record.type === 'expense')
+        .reduce((sum, record) => sum + record.amount, 0);
+    }
+    return expenses;
+  }, [groupedRecords]);
 
   const homeHeaderContent = (
     <div className="header-center-container">
@@ -297,7 +302,7 @@ export default function Home() {
               <div key={date} className="date-group">
                 <div className="date-label">
                   <span>{new Date(date).toLocaleDateString('zh-CN', { month: 'short', day: 'numeric', weekday: 'short' })}</span>
-                  <span className="day-expense">支出 {formatMoney(getDayExpense(date))}</span>
+                  <span className="day-expense">支出 {formatMoney(dayExpenses[date] || 0)}</span>
                 </div>
                 {groupedRecords[date].map((record) => {
                   const icon = categoryIconMap[`${record.type}:${record.category}`] || '📝';