Replace `std::fs::remove_dir_all` with the `Fs` trait's `remove_dir`
method in `remove_root_after_worktree_removal`. This makes the code
testable with `FakeFs` and consistent with the rest of the codebase.

- Add `Arc<dyn Fs>` parameter to `remove_root` and its internal helper
- Use `RemoveOptions { recursive, ignore_if_not_exists }` which handles
both the recursive deletion and NotFound cases cleanly
- Pass `fs` from workspace `AppState` through the sidebar call chain
- Add test verifying error propagation and rollback when `remove_dir`
fails
- Remove tests that were testing raw `std::fs::remove_dir_all` patterns

Release Notes:

- N/A