feat(02-01): implement functional handleSeek

Replace placeholder handleSeek with functional implementation that
calls jamClient.SessionTrackSeekMs(). Uses optimistic UI update for
responsive UX, then seeks the native client. Polling useEffect will
sync actual position within 500ms to correct any drift.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

jam-ui/src/components/client/JKSessionBackingTrackPlayer.js

@@ -153,9 +153,18 @@ const JKSessionBackingTrackPlayer = ({
   };
 
   const handleSeek = async (e) => {
-    // Placeholder for seek functionality
-    // This would need to be implemented based on available jamClient methods
-    console.log('Seek to:', e.target.value);
+    const seekPositionMs = parseInt(e.target.value);
+
+    try {
+      // Update local state immediately for responsive UI
+      setCurrentPositionMs(seekPositionMs);
+      setCurrentTime(formatTime(seekPositionMs));
+
+      // Seek the native client to the new position
+      await jamClient.SessionTrackSeekMs(seekPositionMs);
+    } catch (error) {
+      console.error('Error seeking:', error);
+    }
   };
 
   const handleClose = async () => {