Ghostties v0.1.0-beta.15

Polish and stability fixes following the beta.14 smoke test.

Fixed

• Dark mode titlebar now matches the canvas background color (previously showed a mismatched gray)
• Fullscreen icon position in the toolbar corrected
• Canvas corner radius is now consistent on all four corners
• Shadow depth between the browser panel and terminal panel is now consistent
• Sparkle update-available toast is no longer shown in release builds (debug-only now)

Quality

• All upstream Ghostty tests pass — full test suite is green