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/lib/admin-token-cookie.ts

@@ -0,0 +1,61 @@
+import {
+  ADMIN_TOKEN_COOKIE_MAX_AGE_SECONDS,
+  ADMIN_TOKEN_STORAGE_KEY,
+} from "@/lib/admin-token-constants";
+
+export function readAdminTokenFromCookieString(
+  cookieHeader: string | null | undefined,
+): string | null {
+  if (!cookieHeader) {
+    return null;
+  }
+
+  const prefix = `${ADMIN_TOKEN_STORAGE_KEY}=`;
+  for (const part of cookieHeader.split(";")) {
+    const trimmed = part.trim();
+    if (!trimmed.startsWith(prefix)) {
+      continue;
+    }
+    const raw = trimmed.slice(prefix.length);
+    if (!raw) {
+      return null;
+    }
+    try {
+      const decoded = decodeURIComponent(raw).trim();
+      return decoded !== "" ? decoded : null;
+    } catch {
+      return null;
+    }
+  }
+
+  return null;
+}
+
+export function readAdminTokenFromDocumentCookie(): string | null {
+  if (typeof document === "undefined") {
+    return null;
+  }
+
+  return readAdminTokenFromCookieString(document.cookie);
+}
+
+export function writeAdminTokenCookie(token: string | null): void {
+  if (typeof document === "undefined") {
+    return;
+  }
+
+  const secure =
+    typeof window !== "undefined" && window.location.protocol === "https:";
+  const base = `path=/; SameSite=Lax`;
+
+  if (!token || token.trim() === "") {
+    document.cookie = `${ADMIN_TOKEN_STORAGE_KEY}=; ${base}; max-age=0`;
+    return;
+  }
+
+  const value = encodeURIComponent(token.trim());
+  const maxAge = `max-age=${ADMIN_TOKEN_COOKIE_MAX_AGE_SECONDS}`;
+  document.cookie = `${ADMIN_TOKEN_STORAGE_KEY}=${value}; ${base}; ${maxAge}${
+    secure ? "; Secure" : ""
+  }`;
+}