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/admin/sidebar/SidebarAdmin.tsx

@@ -180,6 +180,16 @@ export default function SidebarAdmin({
     }
   };
 
+  // 放弃未保存修改：回到最近一次保存的布局
+  const discardChanges = () => {
+    if (busy) return;
+    try {
+      setConfig(JSON.parse(savedJsonRef.current));
+    } catch {
+      /* 忽略：快照损坏时保持现状 */
+    }
+  };
+
   /* ---------- 自定义工具 ---------- */
 
   const openCreate = () => {
@@ -304,16 +314,41 @@ export default function SidebarAdmin({
     </nav>
   );
 
+  // 页头与浮动保存条共用的未保存提示 + 操作按钮
+  const dirtyDot = (
+    <span
+      className="inline-block w-2 h-2 rounded-full animate-pulse shrink-0"
+      style={{ background: "var(--gold)" }}
+      aria-hidden
+    />
+  );
+  const saveControls = dirty ? (
+    <div className="flex items-center gap-2">
+      <span className="meta flex items-center gap-1.5 text-[12px]">
+        {dirtyDot}
+        布局有未保存的修改
+      </span>
+      <button type="button" className="btn btn-line btn-sm" disabled={busy} onClick={discardChanges}>
+        撤销修改
+      </button>
+      <button type="button" className="btn btn-primary btn-sm" disabled={busy} onClick={() => void saveConfig()}>
+        {busy ? "保存中…" : "保存布局"}
+      </button>
+    </div>
+  ) : undefined;
+
   return (
     <AdminSettingsPage>
       <AdminPageHeader
         icon={PanelLeft}
         title="侧栏管理"
         description="配置各页面左右侧栏的显示、顺序与位置，管理自定义 HTML 工具"
+        actions={saveControls}
       />
 
       <AdminSettingsWork nav={leftNav}>
-        <div className="space-y-5 pb-24">
+        <>
+          <div className="space-y-5 pb-4">
           {SIDES.map(({ scope, icon, desc, sides }) => {
             const sc = config.scopes[scope] ?? {};
             const left = sc.left ?? [];
@@ -413,27 +448,34 @@ export default function SidebarAdmin({
             </div>
           </section>
         </div>
-      </AdminSettingsWork>
 
-      {/* 吸底保存栏 */}
-      {dirty && (
-        <div
-          className="fixed bottom-0 left-0 right-0 z-40 flex items-center justify-between gap-3 border-t px-4 py-3 sm:px-6"
-          style={{
-            background: "color-mix(in srgb, var(--panel) 92%, transparent)",
-            backdropFilter: "blur(12px)",
-            borderColor: "var(--line)",
-          }}
-        >
-          <p className="meta text-[12px]">
-            <span className="inline-block w-2 h-2 rounded-full mr-1.5" style={{ background: "var(--gold)" }} aria-hidden />
-            布局有未保存的修改
-          </p>
-          <button type="button" className="btn btn-primary" disabled={busy} onClick={() => void saveConfig()}>
-            {busy ? "保存中…" : "保存布局"}
-          </button>
-        </div>
-      )}
+        {/* 浮动保存条：吸在内容列底部居中，滚动全程可见，不再贴屏幕角落 */}
+        {dirty && (
+          <div className="sticky bottom-4 z-30 flex justify-center pointer-events-none">
+            <div
+              className="pointer-events-auto flex items-center gap-3 rounded-full px-4 py-2"
+              style={{
+                background: "color-mix(in srgb, var(--panel) 92%, transparent)",
+                backdropFilter: "blur(12px)",
+                border: "1px solid var(--line)",
+                boxShadow: "var(--shadow-lg)",
+              }}
+            >
+              <span className="meta flex items-center gap-1.5 text-[12px]">
+                {dirtyDot}
+                布局有未保存的修改
+              </span>
+              <button type="button" className="btn btn-line btn-sm" disabled={busy} onClick={discardChanges}>
+                撤销修改
+              </button>
+              <button type="button" className="btn btn-primary btn-sm" disabled={busy} onClick={() => void saveConfig()}>
+                {busy ? "保存中…" : "保存布局"}
+              </button>
+            </div>
+          </div>
+        )}
+        </>
+      </AdminSettingsWork>
 
       {/* 工具编辑弹层 */}
       <Modal