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/lib/utils.js

@@ -0,0 +1,28 @@
+// Date utilities
+export const getMonthStr = (date) =>
+  `${date.getFullYear()}-${String(date.getMonth() + 1).padStart(2, '0')}`;
+
+export const formatMonth = (date) =>
+  `${date.getFullYear()}年${date.getMonth() + 1}月`;
+
+export const formatMoney = (num) => (num || 0).toFixed(2);
+
+// Parse date-time string 'YYYY-MM-DD HH:MM' into date and time parts
+export const parseDateTime = (dateTimeStr) => {
+  if (!dateTimeStr) return { datePart: '', timePart: '00:00' };
+  const [datePart, timePart] = dateTimeStr.split(' ');
+  return { datePart, timePart: timePart || '00:00' };
+};
+
+// Extract date part from 'YYYY-MM-DD HH:MM'
+export const getDatePart = (dateTimeStr) => {
+  if (!dateTimeStr) return '';
+  return dateTimeStr.split(' ')[0];
+};
+
+// Transaction type constants
+export const TRANSACTION_TYPES = {
+  EXPENSE: 'expense',
+  INCOME: 'income',
+  ALL: 'all',
+};