channels: add useAtelierRefresh, for the atelier plugin's Generate/Gallery panels

same stable-bump shape as useFilesRefresh. Generate and Gallery are independent sibling panels with
no props between them (per the panel contract, components only ever receive panelId), so a finished
job reaching the gallery needs a signal rather than the standalone app's onFinished callback prop.

src/workspaces/officerdev/src/channels.ts

@@ -97,3 +97,20 @@ export const useBackgroundJobChanged = () => {
   const bump = useCallback(() => setRef.current(++nonce), []);
   return [signal, bump] as const;
 };
+
+/**
+ * "A generation finished — anyone showing the gallery should re-read."
+ *
+ * The Generate and Gallery panels are independent siblings (see `plugins/atelier/web/panels.ts`) with
+ * no props between them, so a finished job reaching the gallery needs a signal rather than a callback
+ * prop the way the standalone app's `onFinished` was. Same stable-bump shape as `useFilesRefresh`.
+ */
+const useAtelierRefreshChannel = defineChannel<number>('atelier:refresh-signal', 0);
+
+export const useAtelierRefresh = () => {
+  const [signal, setSignal] = useAtelierRefreshChannel();
+  const setRef = useRef(setSignal);
+  setRef.current = setSignal;
+  const bump = useCallback(() => setRef.current(++nonce), []);
+  return [signal, bump] as const;
+};