fix(build): repair the macOS build

[fchorney]

Summary

main does not compile on macOS with stable Rust. Both faults sit inside #[cfg(target_os = "macos")] blocks, which is why CI on the other targets stays green.

Reproduced on a clean worktree at 146d8241, stable rustc 1.96.0, aarch64-apple-darwin. No behaviour change on any target.

1. const fn calling PartialEq (crates/deadlib-renderer/src/window_size.rs)

const fn macos_opengl_low_dpi(backend_type: BackendType, high_dpi: bool) -> bool {
    backend_type == BackendType::OpenGL && !high_dpi
}

error[E0015]: cannot call non-const operator in constant functions

== goes through PartialEq::eq, which is not callable in a const fn on stable (it needs const_trait_impl). Introduced by de4a1c5b.

Fixed with matches!, which pattern-matches instead of calling a trait method, so the function stays const:

matches!(backend_type, BackendType::OpenGL) && !high_dpi

2. Missing trait import (crates/deadsync-shell/src/window.rs)

attributes = attributes.with_disallow_hidpi(!config.high_dpi);

error[E0599]: no method named `with_disallow_hidpi` found for struct `WindowAttributes`

The method exists in winit 0.30.13, but its trait WindowAttributesExtMacOS is not in scope here. The call moved into window.rs during 146d8241 without its import; window_size.rs, where it used to live, still imports the trait. Fixed by importing it, cfg-gated to match the call site.

3. Unused import (crates/deadlib-renderer/src/window_size.rs)

Following from 2, window_size.rs no longer uses WindowAttributesExtMacOS, so on macOS it was an unused import. Dropped.

Testing

cargo check and cargo clippy are clean on macOS with this branch; both fail on main without it. cargo fmt --check clean. The only remaining cargo check warnings are the pre-existing ones in deadsync-audio and deadsync-config.

Noticed while rebasing #616 / #617; those PRs are unaffected by this and do not depend on it.