fix(notifications): make close handler robust against edge cases

[smalluban]

Hardens the notification iframe close path so the popup (most visibly the 30-day review notification) cannot get stuck on screen. Two edge cases could leave the wrapper in the DOM after the user clicked close: (1) after an extension update/reload, the first call in the close handler — chrome.runtime.sendMessage(CLEAR_ACTION) — throws synchronously with Extension context invalidated, aborting the rest of the cleanup; (2) the wrapper was only removed inside a transitionend listener, which never fires under prefers-reduced-motion, hidden tabs, or accessibility tooling that disables CSS transitions.

This PR wraps the CLEAR_ACTION broadcast in try/catch and replaces the transitionend listener with a setTimeout slightly longer than the 0.2s CSS transition, so cleanup always runs. Drive-by: await chrome.storage.local.set in the YouTube notification's dontAsk handler so the value is persisted before the page closes.