Fix/qa defects df002 df011 (#9)

## Summary

What changed and why?

## Change Type

- [ ] Cowork feature
- [ ] Bug fix
- [ ] Core AI contribution
- [ ] Test / hardening
- [ ] Performance
- [ ] Documentation

## Related Work

Cowork Task:

Core Repo: http://34.143.229.138/gitea-admin/fsg-ai-core-assets

Core AI Issue:

Core Task:

Related PR:

## Scope

What is intentionally included?

What is intentionally NOT included?

## Validation

- [ ] Unit tests
- [ ] Integration tests
- [ ] Manual verification
- [ ] Regression check

Commands / evidence:

## Security Impact

Permission / credential / network / customer data impact:

## Compatibility

- [ ] No breaking change
- [ ] Breaking change documented

## Reviewer Notes

Anything Cowork reviewers should pay attention to.

---------

Co-authored-by: thanhnv <thanhnv.ip@gmail.com>
Co-authored-by: Vu Dam Tuan <vudt15@fpt.com>
Reviewed-on: #9

presentation/shell/main_window.py

@@ -212,6 +212,11 @@ class MainWindow(NavRailMixin, RailProjectMixin, TopBarMixin,
             self._set_nav_width_range(_NAV_MIN_WIDTH, self._nav_max_width())
         # Keep the floating Help assistant pinned to the bottom-right corner.
         if getattr(self, "help_agent", None) is not None:
+            # The Cowork composer can wrap an extra control row as the window
+            # narrows/widens, which changes how much bottom guard the dock
+            # needs — recompute it on every resize, not just reposition with
+            # whatever guard height was last measured at tab-entry time.
+            self._update_dock_guard()
             self.help_agent.reposition()
 
     def showEvent(self, event):  # noqa: N802 - Qt override