feat: 前端UI标题栏优化与配置更新

- 更新.gitignore，添加frontend/.next/忽略
- 优化RecordModal、index、recurring、search、stats页面标题栏样式
- 新增frontend/lib/utils.js工具函数
- 更新useCategories.js钩子
- 添加代码优化计划文档

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

frontend/pages/stats.js

@@ -1,12 +1,30 @@
 import { useState, useEffect, useRef, useMemo } from 'react';
 import { useRouter } from 'next/router';
 import { isLoggedIn, getTrendStats, getCategoryStats } from '../lib/api';
-import { Chart, registerables } from 'chart.js';
+import {
+  Chart,
+  LineController,
+  LineElement,
+  PointElement,
+  LinearScale,
+  CategoryScale,
+  Filler,
+  Legend,
+} from 'chart.js';
 import AppShell from '../components/AppShell';
 import { Icon } from '../components/icons';
+import { getMonthStr, formatMonth } from '../lib/utils';
 import { useToast } from '../components/ToastProvider';
 
-Chart.register(...registerables);
+Chart.register(
+  LineController,
+  LineElement,
+  PointElement,
+  LinearScale,
+  CategoryScale,
+  Filler,
+  Legend
+);
 
 export default function Stats() {
   const router = useRouter();
@@ -35,9 +53,6 @@ export default function Stats() {
     loadCategoryData();
   }, [router, currentMonth, categoryType]);
 
-  const getMonthStr = (date) => `${date.getFullYear()}-${String(date.getMonth() + 1).padStart(2, '0')}`;
-  const formatMonth = (date) => `${date.getFullYear()}年${date.getMonth() + 1}月`;
-
   const loadTrendData = async () => {
     setTrendLoading(true);
     try {