Keep the UI attached to background jobs across navigation

Jobs already run on the server — leaving the page or closing the tab does
not stop them — but the SPA lost its link to them because the task id
lived in component state. The online detail page now looks up the newest
task for the post: an active one resumes the progress bar and cancel
button, and a finished one shows "your last download for this post
finished — J-xx". The downloads list accepts a post_id filter for that
lookup.

The footer's Active Workers count is also a link to the staff stats
dashboard, which is the global view of running jobs.

frontend/src/components/AppShell.tsx

@@ -175,7 +175,12 @@ export function AppShell() {
         <Outlet />
       </main>
 
-      <StatusFooter status={status} />
+      <StatusFooter
+        status={status}
+        staff={Boolean(
+          user && (user.is_staff || user.is_superuser || user.role === "staff"),
+        )}
+      />
       <CommandPalette open={paletteOpen} onClose={() => setPaletteOpen(false)} />
     </div>
   );