Fix watched video sync when using browser back button

[rm335]

Summary

• Fix watched videos not syncing when navigating back in Dumpert's new UI
• Add pagehide, pageshow, popstate, and visibilitychange event listeners to cover bfcache and SPA navigation scenarios where beforeunload does not fire

Problem

The extension relied on beforeunload and click events to sync viewed items between sessionStorage and localStorage. Modern browsers use the back-forward cache (bfcache), which suppresses beforeunload. Additionally, Dumpert's new SPA UI uses the History API for navigation, triggering popstate instead of a page unload. This meant watched video state was lost when using the back button.

Solution

Added four additional event listeners:

Event	Covers
pagehide	Page entering bfcache (replacement for beforeunload)
pageshow	Page restored from bfcache via back button
popstate	SPA back/forward navigation (History API)
visibilitychange	Tab losing focus (tab switch, minimize)

Test plan

• Open Dumpert, watch a video, press browser back button — video should remain marked as watched
• Navigate between pages using in-site links — viewed state should persist
• Close and reopen the tab — previously watched videos should still be marked
• Switch tabs and return — viewed state should be synced