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/recurring.js

@@ -1,4 +1,4 @@
-import { useState, useEffect } from 'react';
+import { useState, useEffect, useCallback } from 'react';
 import { useRouter } from 'next/router';
 import { isLoggedIn, getRecurringBills, createRecurringBill, updateRecurringBill, deleteRecurringBill, applyRecurringBill } from '../lib/api';
 import AppShell from '../components/AppShell';
@@ -20,7 +20,7 @@ export default function Recurring() {
     if (categoriesError) {
       showToast('分类加载失败', { tone: 'error' });
     }
-  }, [categoriesError]);
+  }, [categoriesError, showToast]);
   const [bills, setBills] = useState([]);
   const [modalOpen, setModalOpen] = useState(false);
   const [editingBill, setEditingBill] = useState(null);
@@ -38,15 +38,7 @@ export default function Recurring() {
     is_active: true,
   });
 
-  useEffect(() => {
-    if (!isLoggedIn()) {
-      router.push('/login');
-      return;
-    }
-    loadBills();
-  }, [router]);
-
-  const loadBills = async () => {
+  const loadBills = useCallback(async () => {
     setLoading(true);
     try {
       const data = await getRecurringBills();
@@ -57,7 +49,15 @@ export default function Recurring() {
     } finally {
       setLoading(false);
     }
-  };
+  }, [showToast]);
+
+  useEffect(() => {
+    if (!isLoggedIn()) {
+      router.push('/login');
+      return;
+    }
+    loadBills();
+  }, [router, loadBills]);
 
   const openAddModal = () => {
     setEditingBill(null);