修复移动端顶栏折行与头像裁剪弹窗被底部主菜单遮挡

- 主站顶栏在所有断点保持单行：去掉顶栏与动作区的 flex-wrap，品牌和下载、泥点、账户入口不再折成两行
- 窄屏（<640px）下载入口收起为 44×44 图标按钮，保留 aria-label/title，≥640px 恢复「下载客户端」完整文案
- 窄屏（<480px）顶栏品牌只保留 IP 标识，让出空间给下载、泥点与账户入口，并收紧窄屏动作区间距
- 头像裁剪弹窗去掉 portal={false}，改由 UnifiedModal 默认 portal 渲染到 document.body
- 个人中心两级弹窗壳层同步去掉 portal={false}，昵称等个人中心弹窗一并回到页面级层级
- 弹窗不再受 .platform-desktop-layout 的 z-index:1 stacking context 限制，层级高于固定底部主菜单（z-index:60）
- 共享记忆补记单行顶栏口径与「内联弹窗被固定底栏压住」的排障经验

src/components/platform-entry/PlatformEntryActiveFlowShell.tsx

@@ -618,8 +618,8 @@ export function PlatformEntryFlowShellImpl({
             </aside>
 
             <div className="platform-desktop-main flex min-w-0 flex-1 flex-col">
-              <header className="platform-desktop-topbar flex min-h-16 shrink-0 flex-wrap items-center justify-between gap-1 gap-y-2 px-4 py-2 sm:flex-nowrap sm:gap-3 sm:px-6 sm:py-0">
-                <div className="min-w-0 shrink-0 lg:hidden">
+              <header className="platform-desktop-topbar flex min-h-16 shrink-0 flex-nowrap items-center justify-between gap-1 px-4 py-2 sm:gap-3 sm:px-6 sm:py-0">
+                <div className="platform-desktop-topbar__brand min-w-0 shrink-0 lg:hidden">
                   <ActivePlatformBrand />
                 </div>
                 <div className="platform-desktop-topbar__brand-search hidden min-w-0 flex-1 items-center lg:flex">
@@ -662,7 +662,7 @@ export function PlatformEntryFlowShellImpl({
                   </form>
                 </div>
 
-                <div className="platform-desktop-topbar__actions flex max-w-full shrink-0 flex-wrap items-center gap-1 sm:flex-nowrap sm:gap-2 lg:gap-3">
+                <div className="platform-desktop-topbar__actions flex min-w-0 shrink-0 items-center gap-1 sm:gap-2 lg:gap-3">
                   <ClientDownloadEntry />
                   {isAuthenticated ? (
                     <PlatformMudPointWalletEntry