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>

frontend/pages/index.js

@@ -157,9 +157,9 @@ export default function Home() {
       } else {
         const newRecord = await createRecord(data);
         setRecords(prev => [...prev, newRecord]);
-        showToast('记录已添加', { tone: 'success' });
       }
       closeModal();
+      loadData();
     } catch (err) {
       setFormError(err.message);
       showToast(err.message, { tone: 'error' });
@@ -183,6 +183,7 @@ export default function Home() {
       setRecords(prev => prev.filter(r => r.id !== editingRecord.id));
       closeModal();
       showToast('记录已删除', { tone: 'success' });
+      loadData();
     } catch (err) {
       setFormError(err.message);
       showToast(err.message, { tone: 'error' });

frontend/pages/login.js

@@ -1,6 +1,6 @@
 import { useState, useEffect } from 'react';
 import { useRouter } from 'next/router';
-import { login, register, isLoggedIn } from '../lib/api';
+import { login, isLoggedIn } from '../lib/api';
 import AppShell from '../components/AppShell';
 import { Icon } from '../components/icons';
 import { useToast } from '../components/ToastProvider';

frontend/pages/stats.js

@@ -36,7 +36,6 @@ export default function Stats() {
   const [categoryType, setCategoryType] = useState('expense');
   const [selectedCategories, setSelectedCategories] = useState([]);
   const [trendLoading, setTrendLoading] = useState(true);
-  const [categoryLoading, setCategoryLoading] = useState(true);
   const trendChartRef = useRef(null);
   const trendChartInstance = useRef(null);
 
@@ -67,15 +66,12 @@ export default function Stats() {
   };
 
   const loadCategoryData = async () => {
-    setCategoryLoading(true);
     try {
       const category = await getCategoryStats(getMonthStr(currentMonth), categoryType);
       setCategoryData(category);
     } catch (err) {
       console.error(err);
       showToast(err.message, { tone: 'error' });
-    } finally {
-      setCategoryLoading(false);
     }
   };