refactor: 代码质量优化与安全修复

- 删除根目录重复的 server.js (830行)，统一使用模块化的 server/index.js
- 修复 ESLint 错误: 移除未使用变量、修复 const 声明、处理未使用参数
- 修复 React Hooks 依赖警告: 使用 useCallback 包装函数并添加正确依赖项
- SQL LIKE 查询添加特殊字符转义，防止注入风险
- 移除 CSS 中重复的 @keyframes spin 定义

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

frontend/pages/categories.js

@@ -1,4 +1,4 @@
-import { useState, useEffect } from 'react';
+import { useState, useEffect, useCallback } from 'react';
 import { useRouter } from 'next/router';
 import { isLoggedIn, getCategories, createCategory, deleteCategory } from '../lib/api';
 import AppShell from '../components/AppShell';
@@ -30,15 +30,7 @@ export default function Categories() {
     icon: '📝',
   });
 
-  useEffect(() => {
-    if (!isLoggedIn()) {
-      router.push('/login');
-      return;
-    }
-    loadCategories();
-  }, [router]);
-
-  const loadCategories = async () => {
+  const loadCategories = useCallback(async () => {
     setLoading(true);
     try {
       const data = await getCategories();
@@ -49,7 +41,15 @@ export default function Categories() {
     } finally {
       setLoading(false);
     }
-  };
+  }, [showToast]);
+
+  useEffect(() => {
+    if (!isLoggedIn()) {
+      router.push('/login');
+      return;
+    }
+    loadCategories();
+  }, [router, loadCategories]);
 
   const openAddModal = (type) => {
     setCategoryType(type);