fix(ui): enforce must-watch seek lock after the seek settles

Resetting currentTime inside the seeking handler doesn't reliably stick
across sources (HLS re-buffering, emulated YouTube events, coalesced native
seeks), so the snackbar showed but playback continued ahead. The timeupdate
handler then ignored the large forward jump via its delta < 2 guard, making
the skip permanent.

Re-clamp in seeked once the position settles, and add a timeupdate backstop
that snaps back forward jumps (delta >= 2) landing past furthestSeconds.

packages/ui/src/custom/media-player/players/plyr-player.svelte

@@ -116,6 +116,12 @@
 
     const onSeeked = () => {
       isSeeking = false;
+      // Re-enforce once the seek settles: setting currentTime during `seeking`
+      // doesn't always stick (HLS re-buffering can win the in-flight seek), so
+      // snap back here if the player still ended up ahead.
+      if (player.currentTime > furthestSeconds + seekTolerance) {
+        player.currentTime = furthestSeconds;
+      }
       hasBlockedThisSeek = false;
       lastValidTime = player.currentTime;
     };
@@ -126,6 +132,14 @@
       const current = player.currentTime;
       const delta = current - lastValidTime;
 
+      // Backstop: a forward jump beyond normal playback that slipped past the
+      // `seeking`/`seeked` handlers. Clamp back so playback can't continue ahead.
+      if (delta >= 2 && current > furthestSeconds + seekTolerance) {
+        player.currentTime = furthestSeconds;
+        lastValidTime = furthestSeconds;
+        return;
+      }
+
       if (delta > 0 && delta < 2) {
         playedSeconds += delta;
         furthestSeconds = Math.max(furthestSeconds, current);