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/components/ToastProvider.js

@@ -1,5 +1,4 @@
 import { createContext, useCallback, useContext, useMemo, useRef, useState } from 'react';
-import { Icon } from './icons';
 
 const ToastContext = createContext(null);
 
@@ -39,13 +38,7 @@ export function ToastProvider({ children }) {
       <div className="toast-stack" aria-live="polite" aria-atomic="true">
         {toasts.map((toast) => (
           <div key={toast.id} className={`toast toast--${toast.tone}`}>
-            <span className="toast__icon">
-              <Icon name={toast.tone === 'success' ? 'check' : toast.tone === 'error' ? 'close' : 'info'} size={16} />
-            </span>
             <span className="toast__message">{toast.message}</span>
-            <button type="button" className="icon-button icon-button--ghost toast__close" aria-label="关闭提示" onClick={() => removeToast(toast.id)}>
-              <Icon name="close" size={16} />
-            </button>
           </div>
         ))}
       </div>