fix: guard clipboard/eyedropper handlers against updates after unmount

[ngXuanHaiNam612]

Fixes #39

What does this PR do?

Guards 6 clipboard/eyedropper handlers (+ preview-video.tsx) against calling setState / scheduling a new timer after the owning component has unmounted, using a new shared useMountedRef() hook.

Type of change

• Bug fix

Checklist

• npx tsc --noEmit passes
• npm run lint passes
• npm run build passes
• Tested in Chrome (with the html-in-canvas flag, if relevant)

[DavidHDev]

Thanks for taking this on, and for being thorough about it.

The catch is that this repo is on React 19, and since React 18 calling setState on an unmounted component is a silent no-op. The old warning was removed and it isn't a leak, so most of these guards (copy-button, copy-menu, playground-actions, color-picker, html-in-canvas-banner) don't change any behaviour.

Two parts of this are genuinely worth keeping though:

• preview-video.tsx — the cancelled flag is a real fix. Without it, the play().then() callback can pause and rewind the video after the effect has already been cleaned up.
• demo-controls.tsx — guarding play("bloom") is fair, since a sound firing after unmount is actually perceivable.

Would you be up for trimming this down to just those two and dropping the useMountedRef hook? That keeps the real fix without adding a shared abstraction that would need explaining to the next person who reads it.

[ngXuanHaiNam612]

Thanks for the detailed explanation — that's a fair point, and you're right that I conflated "state update after unmount" with "side effect after unmount." I hadn't accounted for React 18+ making unmounted setState a silent no-op.

I'll trim this down to just the two real fixes.