feat: 优化标题栏高度和垂直居中显示

- 调整 AppShell header 高度为 56px 并添加内联样式强制垂直居中
- 优化月份选择器按钮大小为 36px
- 更新 SPEC.md 添加端口配置说明 (前端 3500, 后端 3501)

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

frontend/components/AppShell.js

@@ -33,7 +33,7 @@ export default function AppShell({ title, subtitle, actions, headerContent, chil
 
   return (
     <div className={`app-shell${hideNav ? ' app-shell--standalone' : ''}`}>
-      <header className={headerClassName}>
+      <header className={headerClassName} style={{minHeight: 56, height: 56, padding: 0, display: 'flex', alignItems: 'center'}}>
         <div className="app-shell__container app-header__inner">
           {headerNode ? <div className="app-header__main">{headerNode}</div> : <div className="app-header__spacer" aria-hidden="true" />}
           {actions ? <div className="app-header__actions">{actions}</div> : null}