refactor(layout, i18n, admin): 优化布局结构与多语言支持

调整 AdminShell 组件的子组件顺序，提升代码可读性。更新 admin-breadcrumb 组件，简化导航标签翻译逻辑，确保多语言支持的一致性。重构 admin-language-switcher 组件，优化语言切换的用户体验，增强界面交互性。更新多语言配置，新增登录界面的副标题，提升用户体验。

src/components/admin/admin-language-switcher.tsx

@@ -1,6 +1,6 @@
 "use client";
 
-import { CheckIcon, GlobeIcon } from "lucide-react";
+import { CheckIcon, ChevronDownIcon, GlobeIcon } from "lucide-react";
 import { useEffect, useState } from "react";
 import { useTranslation } from "react-i18next";
 import { toast } from "sonner";
@@ -22,12 +22,6 @@ import {
 } from "@/lib/admin-locale";
 import { cn } from "@/lib/utils";
 
-const LOCALE_FLAGS: Record<AdminApiLocale, string> = {
-  zh: "🇨🇳",
-  en: "🇺🇸",
-  ne: "🇳🇵",
-};
-
 export function AdminLanguageSwitcher() {
   const { i18n, t } = useTranslation("common");
   // Match SSR: do not read document/localStorage until after mount.
@@ -57,25 +51,27 @@ export function AdminLanguageSwitcher() {
     );
   }
 
-  const currentFlag = LOCALE_FLAGS[locale];
+  const currentLabel = t(`language.${locale}`);
 
   return (
     <DropdownMenu>
-      <DropdownMenuTrigger className="inline-flex h-8 items-center gap-1.5 rounded-full border border-slate-200 bg-white px-2 text-left text-slate-700 shadow-[0_1px_2px_rgba(15,23,42,0.04)] outline-none transition hover:border-slate-300 hover:bg-slate-50 focus-visible:ring-2 focus-visible:ring-ring">
-        <span className="flex size-5 shrink-0 items-center justify-center rounded-full bg-slate-100 text-xs">
-          {currentFlag}
-        </span>
+      <DropdownMenuTrigger
+        aria-label={t("language.title")}
+        className="inline-flex h-9 max-w-[9rem] items-center gap-1.5 rounded-lg px-2.5 text-sm font-medium text-foreground outline-none transition-colors hover:bg-muted/80 focus-visible:ring-2 focus-visible:ring-ring"
+      >
         <GlobeIcon
-          className="size-4 shrink-0 stroke-[1.75] text-slate-400 sm:hidden"
+          className="size-4 shrink-0 text-muted-foreground"
+          aria-hidden
+        />
+        <span className="min-w-0 truncate">{currentLabel}</span>
+        <ChevronDownIcon
+          className="size-3.5 shrink-0 text-muted-foreground/80"
           aria-hidden
         />
       </DropdownMenuTrigger>
-      <DropdownMenuContent
-        align="end"
-        className="w-[188px] overflow-hidden rounded-xl border border-slate-200 bg-white p-1 shadow-[0_16px_40px_rgba(15,23,42,0.12)]"
-      >
-        <DropdownMenuGroup className="space-y-0.5">
-          <DropdownMenuLabel className="sr-only">
+      <DropdownMenuContent align="end" className="min-w-[10.5rem]">
+        <DropdownMenuGroup>
+          <DropdownMenuLabel className="text-xs text-muted-foreground">
             {t("language.title")}
           </DropdownMenuLabel>
           {ADMIN_API_LOCALES.map((code) => {
@@ -85,29 +81,17 @@ export function AdminLanguageSwitcher() {
               <DropdownMenuItem
                 key={code}
                 className={cn(
-                  "flex min-h-[42px] items-center gap-2 rounded-md border border-transparent px-2 py-1.5 text-slate-700 outline-none transition",
-                  active
-                    ? "border-rose-100 bg-rose-50 text-rose-600"
-                    : "hover:bg-slate-50 focus:bg-slate-50",
+                  "flex items-center justify-between gap-2",
+                  active && "bg-accent text-accent-foreground",
                 )}
                 onClick={() => void onSelectLocale(code)}
               >
-                <span className="flex size-8 shrink-0 items-center justify-center rounded-md bg-white text-lg shadow-[inset_0_0_0_1px_rgba(148,163,184,0.16)]">
-                  {LOCALE_FLAGS[code]}
-                </span>
-                <span className="min-w-0 flex-1">
-                  <span
-                    className={cn(
-                      "block truncate text-[14px] font-semibold leading-5",
-                      active ? "text-rose-600" : "text-slate-800",
-                    )}
-                >
+                <span className="truncate font-medium">
                   {ADMIN_LOCALE_LABELS[code]}
                 </span>
-                </span>
-                <span className="flex w-3 shrink-0 justify-end">
-                  {active ? <CheckIcon className="size-3.5 text-rose-500" /> : null}
-                </span>
+                {active ? (
+                  <CheckIcon className="size-4 shrink-0 text-primary" />
+                ) : null}
               </DropdownMenuItem>
             );
           })}