fix(frontend): refresh file storage stats on tab and page revisit

Set stats query staleTime to 0 with refetchOnMount always, and unmount
inactive file admin tabs so returning to the stats view triggers a new fetch
instead of serving the 30s React Query cache.

frontend/app/(main)/admin/files/page.tsx

@@ -42,16 +42,16 @@ export default function FilesPage() {
 
         {/* ──────── TAB 1: 统计看板 ──────── */}
         <TabsContent value="stats" className="outline-hidden">
-          <FileStats />
+          {activeTab === "stats" ? <FileStats /> : null}
         </TabsContent>
 
         {/* ──────── TAB 2: 文件列表 ──────── */}
         <TabsContent value="list" className="outline-hidden">
-          <FileList />
+          {activeTab === "list" ? <FileList /> : null}
         </TabsContent>
 
         <TabsContent value="storage" className="outline-hidden">
-          <StorageConfigTab />
+          {activeTab === "storage" ? <StorageConfigTab /> : null}
         </TabsContent>
       </Tabs>
     </motion.div>