title_bar: Only draw the window controls the WM supports (#2641)

`WindowControls` draws minimize and maximize on every non-macOS
platform, whether or not the window manager can honor them. On niri — a
scrollable-tiling Wayland compositor that advertises maximize but not
minimize — that leaves a minimize button that does nothing, and an app
using `TitleBar` has no way to opt out.

GPUI already knows the answer: it parses xdg-shell's `wm_capabilities`
into `Window::window_controls()`. Zed's own title bar filters its
buttons on exactly that
(`crates/platform_title_bar/src/platforms/platform_linux.rs`). This does
the same, and keeps close unconditional.

## Effect

- **Wayland** — the buttons follow what the compositor advertises.
- **X11, Windows, macOS** — unchanged: `WindowControls::default()` is
all-true, and macOS still returns early.

No API change.

## Testing

Built and run on NixOS under niri, in an app using `TitleBar`:

- close — shown, works
- maximize — shown, works, including restore
- minimize — no longer drawn (niri does not advertise it)

Also built and run on macOS, where the path is unchanged, and `cargo
test -p gpui-component` passes (348 tests).

## AI assistance

Per CONTRIBUTING: this patch was proposed and written by Claude
(Anthropic), which found `window_controls()` and the Zed precedent while
we were building a desktop app on this kit. Reviewed and tested by me.

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>