fix: enable compilation on macOS (Darwin) for testing and CI

[levonk]

Summary

Fix compilation errors on macOS (Darwin) by adding platform-specific guards for Wayland-only APIs.

Current problem

The codebase has unguarded Wayland-specific imports and method calls that prevent compilation on macOS:

• winit::platform::wayland::WindowAttributesExtWayland import
• with_name() method calls on WindowAttributes

This blocks:

• Running tests on macOS in CI
• Developers on macOS from building/test the codebase
• Cross-platform development workflows

Root cause

handterm is Wayland-native, but the code doesn't guard Linux/Wayland-specific code with #[cfg(target_os = "linux")].

Proposed fix

Add #[cfg(target_os = "linux")] guards around:

1. Wayland platform imports in src/app.rs, src/gpu_runtime.rs, src/remote_app.rs
2. with_name() method calls in window attribute builders

Implementation

Fixed in commit: (add commit hash after commit)

Changes:

• src/app.rs: guarded Wayland import and with_name() call
• src/gpu_runtime.rs: guarded Wayland import and with_name() call
• src/remote_app.rs: guarded Wayland import and with_name() call

Testing

After fixes:

• 211 tests pass on macOS
• 10 font-related tests fail (pre-existing macOS font configuration issues, not compilation errors)
• Compilation succeeds on macOS

Notes

handterm will still not run on macOS (requires Wayland compositor), but this enables:

• CI testing on macOS
• Development builds for testing non-Wayland code paths
• Cross-platform collaboration

The app will gracefully fail at runtime on macOS with a clear error about Wayland requirement.