fix: memory leak due to missing unsubscribe (#12777)

- unsubscribe `Signal` not correctly
- missing un-subscription for `Livedata.signal`

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

- **Bug Fixes**
- Improved resource management to ensure subscriptions are properly
cleaned up, reducing potential memory leaks and improving overall app
stability.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

blocksuite/affine/widgets/toolbar/src/toolbar.ts

@@ -541,9 +541,7 @@ export class AffineToolbarWidget extends WidgetComponent {
           );
         });
 
-        return () => {
-          subscription.unsubscribe();
-        };
+        disposables.add(subscription);
       })
     );