### Objective

PR #36859 made native macOS and Windows controls respect `is_resizable`
and `is_minimizable`, but custom window decorations cannot access those
values from `Window`. Linux custom controls and client-side resize
interactions remain active, macOS custom titlebar double-click does not
check either option and custom Windows caption buttons still appear
enabled even though the backend blocks their actions.

### Solution

Expose `is_resizable` and `is_minimizable` on `Window`. Use them to
disable custom window controls and titlebar actions and omit client-side
resize cursors and hitboxes for non-resizable windows.

## Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [ ] Unsafe blocks (if any) have justifying comments
- [x] The content adheres to Zed's UI standards
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [ ] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- N/A