feat: 品牌标识用云+数据库矢量图标（favicon + 页眉品牌标）

- Icons.jsx 新增 CloudDbIcon：照 icons8-云 重绘为矢量，左云随主题色、右圆柱用品牌蓝
- GlobalHeader 品牌区前置该图标，原「系统在线」绿点改为挂在其右下角
- index.html 增加 rel=icon / mask-icon 指向 public/favicon.svg
- luxury.css 补品牌样式，并让 :not() 选择器排除 .brand-mark 避免样式误伤

frontend/src/components/GlobalHeader.jsx

@@ -1,7 +1,7 @@
 import React, { useEffect, useMemo, useRef, useState } from 'react'
 import { useNavigate } from 'react-router-dom'
 import axios from 'axios'
-import { BotIcon, ClockIcon, PlusCircleIcon, SearchIcon } from './Icons'
+import { BotIcon, ClockIcon, CloudDbIcon, PlusCircleIcon, SearchIcon } from './Icons'
 
 function GlobalHeader({ onOpenAssistant }) {
   const [now, setNow] = useState(new Date())
@@ -49,7 +49,7 @@ function GlobalHeader({ onOpenAssistant }) {
 
   return (
     <header className="global-header">
-      <div className="global-header-brand"><span className="header-status-dot" /><div><strong>happy出口通</strong><span>EXPORT OPERATIONS CONSOLE</span></div></div>
+      <div className="global-header-brand"><span className="brand-mark" title="happy出口通"><CloudDbIcon size={28} /><span className="header-status-dot" /></span><div><strong>happy出口通</strong><span>EXPORT OPERATIONS CONSOLE</span></div></div>
       <div className={`global-search ${searchOpen ? 'is-open' : ''}`}>
         <SearchIcon size={17} />
         <input ref={searchRef} aria-label="搜索 everything" value={query} onFocus={() => setSearchOpen(true)} onChange={event => setQuery(event.target.value)} placeholder="搜索 everything" autoComplete="off" />