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/components/admin/AdminShell.tsx

@@ -28,6 +28,7 @@ import {
   BarChart3,
   Award,
   Layers,
+  Images,
 } from "lucide-react";
 import type { User } from "@/lib/api";
 import { apiAdminPendingCounts, apiLogout } from "@/lib/api";
@@ -62,7 +63,7 @@ interface NavGroup {
   items: NavItem[];
 }
 
-// 分组导航：概览 → 内容运营 → 成员经济 → 站点配置
+// 分组导航：概览 → 内容 → 成员与经济 → 外观展示 → 系统
 const NAV_GROUPS: NavGroup[] = [
   {
     title: "概览",
@@ -74,6 +75,12 @@ const NAV_GROUPS: NavGroup[] = [
   {
     title: "内容",
     items: [
+      {
+        href: "/admin/boards",
+        label: "板块管理",
+        icon: Columns3,
+        visible: (u) => isOwner(u.role),
+      },
       {
         href: "/admin/content",
         label: "内容管理",
@@ -99,12 +106,6 @@ const NAV_GROUPS: NavGroup[] = [
         icon: FileText,
         visible: (u) => isAdminOrAbove(u.role),
       },
-      {
-        href: "/admin/ads",
-        label: "推广与赞助",
-        icon: CircleDollarSign,
-        visible: (u) => isAdminOrAbove(u.role),
-      },
     ],
   },
   {
@@ -134,23 +135,17 @@ const NAV_GROUPS: NavGroup[] = [
         icon: Award,
         visible: (u) => isSuperOrOwner(u.role),
       },
+      {
+        href: "/admin/ads",
+        label: "推广与赞助",
+        icon: CircleDollarSign,
+        visible: (u) => isAdminOrAbove(u.role),
+      },
     ],
   },
   {
-    title: "站点",
+    title: "外观展示",
     items: [
-      {
-        href: "/admin/boards",
-        label: "板块管理",
-        icon: Columns3,
-        visible: (u) => isOwner(u.role),
-      },
-      {
-        href: "/admin/settings",
-        label: "站点设置",
-        icon: Globe,
-        visible: (u) => isSuperOrOwner(u.role),
-      },
       {
         href: "/admin/appearance",
         label: "外观",
@@ -163,6 +158,23 @@ const NAV_GROUPS: NavGroup[] = [
         icon: PanelLeft,
         visible: (u) => isSuperOrOwner(u.role),
       },
+      {
+        href: "/admin/media",
+        label: "媒体库",
+        icon: Images,
+        visible: (u) => isSuperOrOwner(u.role),
+      },
+    ],
+  },
+  {
+    title: "系统",
+    items: [
+      {
+        href: "/admin/settings",
+        label: "站点设置",
+        icon: Globe,
+        visible: (u) => isSuperOrOwner(u.role),
+      },
     ],
   },
 ];