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/NotificationBell.tsx

@@ -201,31 +201,46 @@ export default function NotificationBell({ initialUnread = 0 }: { initialUnread?
       </button>
 
       {open && (
-        <div
-          className="absolute right-0 mt-2.5 w-[340px] max-w-[calc(100vw-2rem)] overflow-hidden z-50 j13-toast-in"
-          style={{
-            background: "var(--panel)",
-            border: "1px solid var(--line)",
-            borderRadius: "var(--r)",
-            boxShadow: "var(--shadow-lg)",
-          }}
-        >
+        <>
+          {/* 移动端遮罩：点击关闭（桌面不渲染） */}
           <div
-            className="flex items-center justify-between px-5 py-3.5"
-            style={{ borderBottom: "1px solid var(--line)" }}
+            className="lg:hidden fixed inset-0 z-40"
+            style={{ background: "rgba(0,0,0,0.35)" }}
+            onClick={() => setOpen(false)}
+            aria-hidden="true"
+          />
+          <div
+            className="absolute right-0 mt-2.5 w-[340px] max-w-[calc(100vw-2rem)] overflow-hidden z-50 j13-toast-in flex flex-col
+              max-lg:fixed max-lg:inset-x-0 max-lg:bottom-0 max-lg:top-auto max-lg:mt-0 max-lg:w-full max-lg:max-w-none max-lg:max-h-[75dvh] max-lg:rounded-t-2xl!"
+            style={{
+              background: "var(--panel)",
+              border: "1px solid var(--line)",
+              borderRadius: "var(--r)",
+              boxShadow: "var(--shadow-lg)",
+            }}
           >
-            <span className="font-semibold text-[15px]" style={{ color: "var(--ink)" }}>通知</span>
-            {unread > 0 && (
-              <button
-                onClick={handleMarkAllRead}
-                className="flex items-center gap-1 text-xs meta hover:text-[var(--accent)] transition-colors"
-              >
-                <CheckCheck size={13} /> 全部已读
-              </button>
-            )}
-          </div>
+            {/* 移动端底部面板抓手 */}
+            <div
+              className="lg:hidden mx-auto mt-2 mb-[-6px] w-10 h-1 rounded-full shrink-0"
+              style={{ background: "var(--line-2)" }}
+              aria-hidden="true"
+            />
+            <div
+              className="flex items-center justify-between px-5 py-3.5 shrink-0"
+              style={{ borderBottom: "1px solid var(--line)" }}
+            >
+              <span className="font-semibold text-[15px]" style={{ color: "var(--ink)" }}>通知</span>
+              {unread > 0 && (
+                <button
+                  onClick={handleMarkAllRead}
+                  className="flex items-center gap-1 text-xs meta hover:text-[var(--accent)] transition-colors"
+                >
+                  <CheckCheck size={13} /> 全部已读
+                </button>
+              )}
+            </div>
 
-          <div className="max-h-[24rem] overflow-y-auto">
+            <div className="flex-1 min-h-0 max-h-[24rem] max-lg:max-h-none overflow-y-auto">
             {loading ? (
               <p className="text-center py-10 meta">加载中...</p>
             ) : notifications.length === 0 ? (
@@ -303,7 +318,7 @@ export default function NotificationBell({ initialUnread = 0 }: { initialUnread?
           </div>
 
           <div
-            className="px-4 py-2.5 text-center"
+            className="px-4 py-2.5 text-center shrink-0"
             style={{ borderTop: "1px solid var(--line)" }}
           >
             <Link
@@ -314,7 +329,8 @@ export default function NotificationBell({ initialUnread = 0 }: { initialUnread?
               查看全部通知 →
             </Link>
           </div>
-        </div>
+          </div>
+        </>
       )}
     </div>
   );