fix(#2443): address asyncBtn review notes

- asyncBtn now returns fn().finally(...) so callers can await/chain it
- Move re-fetch calls inside try/catch in core.js and permissions.js so
  network errors from fetchAndRenderStalePerms / fetchAndRender* are
  caught instead of escaping as unhandled rejections
- clearStaleAgent returns the asyncBtn promise so the function is
  properly awaitable when a button is present
- Update asyncBtn doc comment to reflect the return-value contract

frontend/packages/dashboard/src/core.js

@@ -115,12 +115,11 @@ function renderStalePerms(root, ghosts) {
           errP.hidden = false;
           return;
         }
+        await fetchAndRenderStalePerms();
       } catch (err) {
         errP.textContent = 'failed to clear perms for ' + name + ': ' + err;
         errP.hidden = false;
-        return;
       }
-      await fetchAndRenderStalePerms();
     }));
     li.append(btn);
     ul.append(li);