fix(ui): restore contrast to editor controls

[DavidBabinec]

What changed

• match the canvas mode toggle shell to the canvas/body background
• restore the shared Switch primitive's high-contrast checked, unchecked, and disabled states
• add regression coverage for both token choices

Why

The canvas mode toggle was painted with a raised surface color instead of the black canvas chrome. Separately, a token migration had collapsed the Switch checked track onto a near-identical surface tone, making its state difficult to read on editor cards.

Impact

Canvas chrome now blends into the intended background, while switches remain clearly distinguishable across dark and light editor surfaces.

Verification

• bun test src/__tests__/ui/neutralInteractiveChrome.test.ts src/__tests__/canvas/canvasNotch.test.ts src/__tests__/canvas/canvasMode.test.tsx (29 passing)
• bun run lint
• bun run build
• disposable local browser pass covering both checked and unchecked switch states