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/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);
     }
   };