feat: global css variables and app shell

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/components/BootScreen/BootScreen.tsx

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+export default function BootScreen({ onComplete }: { onComplete: () => void }) { return <div onClick={onComplete}>Loading...</div>; }

src/components/Desktop/Desktop.tsx

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+export default function Desktop() { return <div>Desktop</div>; }