Closes #59235

When a panel's `default_size` changes (e.g. `git_panel.default_width` is
edited in settings), the persisted per-workspace size is now cleared so
the new default takes effect immediately. Previously the saved dragged
size took precedence forever, making `default_width` effectively useless
after the first manual resize.

This fix is at the Dock level (`crates/workspace/src/dock.rs`), so it
applies to all panels — Git Panel, Project Panel, Terminal Panel, etc.

Release Notes:

- Fixed panel `default_width` setting being ignored after manually
resizing the panel


https://github.com/user-attachments/assets/97ef69b2-387f-4e29-bc4c-0f336f24eb0b