Match macOS chrome and branding to dark mode

[wisdomqin]

What changed

• keep the macOS traffic-light strip dark in both Harness light and dark appearances
• use separate static sidebar assets: black in light mode and white in dark mode
• set the expanded sidebar top clearance to 22px so the selected brand area clears the macOS traffic lights without sitting too low
• replace the desktop system icon with a rounded black tile and white DSH whale on macOS and Windows
• follow Harness's authoritative body[data-ds-dark-theme] state so branding updates immediately without restarting
• install a dedicated transparent brand asset into the Harness frontend
• preserve the original Harness fish on the central welcome screen

Why

Harness controls its own appearance independently from the macOS system theme. A one-time startup check left the sidebar logo stale after changing appearance, while allowing the native chrome to follow the page could turn the traffic-light strip white.

The desktop shell now separates these responsibilities: native macOS chrome remains dark, while the sidebar reads Harness's own resolved theme state directly. No duplicate color sampling or page-level theme observer can override the logo.

User impact

• light mode shows the black DSH Desktop logo
• dark mode shows the white DSH Desktop logo
• the welcome screen keeps the original theme-aware Harness fish without a blue square background
• the expanded sidebar logo has additional spacing below the traffic-light area
• Dock, Finder and Windows use the rounded black-and-white application icon
• the macOS top strip remains dark in both modes

Validation

• npm run typecheck
• npm test (40 tests)
• npm run build
• git diff --check
• visually verified in both light and dark modes in the running macOS development build
• visually verified a light → dark → light cycle without restarting the development build