feat: implement sprint 1 quick wins - toast system, notifications, UX polish

- Add central toast notification system (popup.html, popup.js)
- Add browser notifications toggle (opt-in) with event toasts
- Fix interpolation memory leak (unload listener)
- Add /health endpoint with IP-based rate limiting (server)
- Improve tab sorting (current tab first, matches, alphabetical)
- Add copy-to-clipboard visual feedback with toast
- Show targetTime in last action card for seek/force sync
- Add explicit video cleanup when element removed (content.js)
- Update ROADMAP.md to remove implemented features

extension/content.js

@@ -507,6 +507,13 @@
     function checkVideo() {
         lastMutate = Date.now();
         const video = findVideo();
+
+        if (!video && lastVideoSrc) {
+            reportLog('Video element removed from page', 'warn');
+            lastVideoSrc = null;
+            return;
+        }
+
         if (!video) return;
 
         const currentSrc = video.currentSrc || video.src;