Remove UI density setting

[jackwener]

Summary

• remove the persisted UI density setting from core settings and strip legacy appearance.density during normalization
• delete the renderer density state, applyDensity helper, Settings density picker, CSS density token switch, and dead density swatch styles
• keep Appearance order as personalization before theme and update docs/contracts to say density is not a product setting

Verification

• npm --workspace @maka/desktop run typecheck
• npm --workspace @maka/core test
• npm --workspace @maka/desktop test -- settings-theme-contract renderer-startup-fail-soft-contract settings-form-a11y-contract (actual full desktop 1466/1466; check-console/check-a11y clean)
• npm run build (passes; existing Vite chunk-size warning)
• git diff --check