Hoist usePollSubscriptions to RoomWidgetsView; drop the side effect from usePollWidget

Follow-up to the previous commit's poll split. The compat shim
usePollWidget used to call usePollSubscriptions() inside its body so
the three RoomSessionPollEvent listeners were still registered for
existing consumers — but that meant:
- listeners would be re-registered per consumer (today nobody, since
  useWordQuizWidget was already migrated to usePollActions);
- the lifetime of the subscriptions was tied to a leaf widget instead
  of the room session;
- a render of a component using the shim had the side effect of
  attaching three global event listeners.

Move
- src/components/room/widgets/RoomWidgetsView.tsx now calls
  usePollSubscriptions() once at the top of the room-widget tree. The
  bridge from RoomSessionPollEvent (OFFER/ERROR/CONTENT) to the UI
  event bus is now mounted for exactly the lifetime of an in-room
  session, regardless of which leaf widget renders.
- src/hooks/rooms/widgets/usePollWidget.ts (compat shim) is reduced
  to a one-liner that just returns usePollActions(). It is still
  deprecated; remove once nothing imports it.

Verification
- yarn eslint on the two touched files: 1 pre-existing error
  (the same FC<{}> in RoomWidgetsView that was there before — baseline
  unchanged; I deliberately did not touch it in this commit to keep
  the diff minimal).
- yarn test: 22/22 still passing.
- grep confirms usePollWidget has zero in-tree consumers; the only
  importer is the barrel re-export.

src/hooks/rooms/widgets/usePollWidget.ts

@@ -1,17 +1,12 @@
 import { usePollActions } from './usePollActions';
-import { usePollSubscriptions } from './usePollSubscriptions';
 
 /**
- * @deprecated Prefer `usePollSubscriptions` (mount once, top-level) and
- * `usePollActions` (anywhere a component dispatches a vote/accept/reject).
- * This shim preserves the old `{ startPoll, rejectPoll, answerPoll }`
- * shape for existing consumers, but each call also re-mounts the three
- * subscription listeners — which is wrong if the hook is called from
- * multiple places.
+ * @deprecated Prefer `usePollActions` for components that dispatch
+ * votes/accepts/rejects. The corresponding subscriptions are now mounted
+ * once by `RoomWidgetsView` via `usePollSubscriptions`, so this shim no
+ * longer needs to register them transitively.
+ *
+ * Kept only to preserve the `{ startPoll, rejectPoll, answerPoll }`
+ * shape for any out-of-tree consumer; remove once nothing imports it.
  */
-export const usePollWidget = () =>
-{
-    usePollSubscriptions();
-
-    return usePollActions();
-};
+export const usePollWidget = () => usePollActions();