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-nav-label.ts

@@ -0,0 +1,38 @@
+import type { TFunction } from "i18next";
+
+/** 与 {@link AdminBreadcrumb} / 侧栏共用：仅用 i18n，避免 API 旧 label 作 defaultValue 导致水合不一致 */
+const NAV_SEGMENT_I18N_KEYS: Record<string, string> = {
+  dashboard: "dashboard",
+  admin_users: "admin_users",
+  admin_roles: "admin_roles",
+  players: "players",
+  currencies: "currencies",
+  wallet: "wallet",
+  draws: "draws",
+  rules_plays: "rules_plays",
+  rules_odds: "rules_odds",
+  jackpot: "jackpot",
+  risk_cap: "risk_cap",
+  risk: "risk",
+  settlement: "settlement",
+  reconcile: "reconcile",
+  reports: "reports",
+  tickets: "tickets",
+  audit: "audit",
+  settings: "settings",
+  integration: "integration",
+  config: "config",
+};
+
+export function adminNavLabel(
+  segment: string,
+  t: TFunction,
+  apiLabel?: string | null,
+): string {
+  const key = NAV_SEGMENT_I18N_KEYS[segment];
+  if (key) {
+    return t(`nav.${key}`, { ns: "common" });
+  }
+
+  return apiLabel?.trim() ? apiLabel : segment;
+}