fix(frontend): clear sidebar update dot after toolbar Update click (#763)

The toolbar Update button calls updateStack(), which refreshed
containers and stacks but never re-fetched the image-updates list.
The sidebar's blue "update available" dot therefore stayed visible
until the 5-minute polling interval. The right-click context menu
path (executeStackActionByFile, action='update') already calls
fetchImageUpdates() on success; mirror that call here so both paths
behave the same.

frontend/src/components/EditorLayout.tsx

@@ -1431,6 +1431,7 @@ export default function EditorLayout() {
         throw new Error(errText || 'Update failed');
       }
       toast.success('Stack updated successfully!');
+      fetchImageUpdates();
       // Refresh containers after update
       if (selectedFile === stackFile) {
         const containersRes = await apiFetch(`/stacks/${stackName}/containers`);