fix(gui): make titlebar options platform-aware for Wayland compatibility

[penso]

Summary

• Gate macOS-specific titlebar options (appears_transparent: true, traffic_light_position) behind #[cfg(target_os = "macos")] so they don't interfere with Wayland server-side decoration negotiation on Linux
• Extract default_titlebar_options() helper in constants.rs to centralize platform-aware titlebar configuration and eliminate duplication across both window creation sites

Closes #68

Why this approach

The root cause is appears_transparent: true interfering with how Wayland compositors handle server-side decorations (resize handles, drag, close/min/max buttons). Making it false on non-macOS platforms fixes the issue without needing to force X11 via environment variable hacks.

Test plan

• just format — passes
• just lint — zero warnings
• just test — all tests pass
• Verify on Ubuntu 25.04 (Wayland/GNOME): window has resize handles, drag, and close/min/max buttons
• Verify macOS is unaffected (same appears_transparent: true + traffic_light_position as before)

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[codspeed-hq]

Merging this PR will not alter performance

✅ 21 untouched benchmarks

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_{Comparing claude/nice-saha (9365e2d) with main (c056db5)¹}

Footnotes

1. No successful run was found on main (9365e2d) during the generation of this report, so c056db5 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩