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/components/RecordModal.js

@@ -24,11 +24,8 @@ export default function RecordModal({
   return (
     <div className="modal-overlay modal-overlay--fullscreen" onClick={onClose}>
       <div className="modal modal--fullscreen" onClick={(e) => e.stopPropagation()}>
-        <div className="modal-header">
-          <div>
-            <p className="eyebrow">{editingRecord ? '编辑记录' : '新增记录'}</p>
-            {editingRecord ? <h2>更新账目</h2> : null}
-          </div>
+        <div className="modal-header" style={{minHeight: 56, height: 56, padding: 'calc(var(--safe-top) + 8px) 16px 8px', display: 'flex', alignItems: 'center', justifyContent: 'space-between', boxSizing: 'border-box'}}>
+          <h2 style={{margin: 0, fontSize: '1.1rem'}}>{editingRecord ? '编辑记录' : '新增记录'}</h2>
           <button
             type="button"
             className="icon-button icon-button--ghost"