feat: 站点媒体库与移动端底部导航，服务测试补强

- 新增媒体库：media_library/media_thumbs 服务（WebP 缩略图）、管理端 media 页面
- 移动端底部导航 MobileTabBar 替换 MobilePostBar
- 旧数据导入增强与测试、路由与登录会话/板块侧边栏测试补强
- site-doc 组件精简（移除 Breadcrumb），文档新增迁移公告说明

frontend/app/layout.tsx

@@ -1,5 +1,5 @@
 import OperationalBanner from "@/components/OperationalBanner";
-import type { Metadata } from "next";
+import type { Metadata, Viewport } from "next";
 import { cookies, headers } from "next/headers";
 import { Plus_Jakarta_Sans, Noto_Sans_SC, JetBrains_Mono } from "next/font/google";
 import "./globals.css";
@@ -76,6 +76,14 @@ export function generateMetadata(): Metadata {
   };
 }
 
+// 移动端键盘弹出时压缩布局高度（而非仅视觉平移），聊天输入框随 100dvh
+// 收缩保持可见，配合聊天页锁视口布局；再叠加聚焦时 scrollIntoView 兜底
+export const viewport: Viewport = {
+  width: "device-width",
+  initialScale: 1,
+  interactiveWidget: "resizes-content",
+};
+
 export default async function RootLayout({
   children,
 }: {