fix: clean up closed viewers when transitions are interrupted

[infeng]

Closing a viewer can leave an invisible overlay blocking the page when its CSS transition is disabled, interrupted, or never starts. The close path depended entirely on transitionend.

Related to #122. Keep the fade-out, stop intercepting pointer events after closing, and hide the viewer with a fallback based on the computed opacity transition duration and delay. Cancel that fallback on reopen/unmount and accept only the root opacity transition event.

Validation:

• Regression failed before the fix; all 46 tests and npm run verify now pass, including library/demo/SSR builds, hydration, multipart, package checks, and both critical audit gates.
• Chrome 149 reproduced disabled/cancelled/rapid-close failures before the fix. After the fix, all three and normal close passed three open/close cycles with display: none and successful page hit tests.
• Browser checks also passed custom 800ms duration + 100ms delay, reopening before cleanup, and inline closing with transitions disabled.
• Registry scan passed; no dependency changes. Keep the issue open for release tracking.