Fix macOS sidebar chrome and rail layout

[wisdomqin]

What

• Remove the transient injected macOS titlebar strip and match the native window background to Harness's resolved theme before showing the window.
• Tighten the DSH logo and original Harness wordmark spacing.
• Align expanded and collapsed sidebar header spacing with the macOS traffic lights.
• Use an 80px collapsed rail on macOS while retaining the upstream 56px rail on other platforms.
• Add regression coverage for initial theme presentation and macOS rail geometry.

Why

The previous first-load titlebar injection produced a black strip in light mode and double-counted vertical sidebar spacing. The default 56px rail was also narrower than the macOS traffic-light group.

User impact

The first rendered frame now matches the selected light or dark theme, sidebar branding keeps stable placement before and after refresh, and the collapsed macOS rail clears the traffic lights while keeping its controls centered.

Checks

• Fresh npm ci with all patches applied
• npm test — 41 tests passed
• npm run typecheck
• npm run build
• git diff --check