feat(window): adopt native macos chrome

[cododel]

Summary

Adopts native macOS window chrome while preserving the existing custom CanvasTTY title bar on Windows and Linux.

UI changes

• Adds a compact 32px macOS brand bar with space reserved for native traffic lights.
• Preserves the existing custom title bar and right-side window controls on Windows and Linux.
• Updates startup and failure screens to match the macOS layout from the first frame.

Native macOS controls

• Uses Electron titleBarStyle: "hidden" on macOS.
• Positions native traffic lights at x: 12, y: 10.
• Removes renderer-owned macOS traffic lights and fullscreen controls.
• Leaves macOS responsible for close, minimize, maximize, fullscreen, and fullscreen hover behavior.

Platform detection

• Detects macOS in the main process before window creation, so native chrome applies during the loading screen.
• Exposes synchronous isMacOS through preload for the renderer’s initial layout.
• Uses fullscreen and maximize events only to synchronize runtime layout.
• Does not rely on browser user-agent or navigator.platform.

Fullscreen fix

• Removes the custom fullscreen hover layer and CSS layout workaround.
• Hides the renderer brand bar in native macOS fullscreen.
• Restores it after the native leave-full-screen event.
• Handles both the native green button and Control + Command + F.

Validation

• Added coverage for platform-specific window chrome options.
• Added coverage for native fullscreen state events and the preload contract.
• Added startup-page coverage for native traffic-light spacing.
• npm test — 214 passing.
• npm run build — passing.