fix: restore border between track controls and waveforms

Added back the right border on the track controls column to visually
separate the controls from the waveform area.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

components/tracks/TrackList.tsx

@@ -100,7 +100,7 @@ export function TrackList({
       {/* Track List - Two Column Layout */}
       <div className="flex-1 flex overflow-hidden">
         {/* Left Column: Track Controls (Fixed Width, No Scroll - synced with waveforms) */}
-        <div ref={controlsScrollRef} className="w-48 flex-shrink-0 overflow-hidden pb-3">
+        <div ref={controlsScrollRef} className="w-48 flex-shrink-0 overflow-hidden pb-3 border-r border-border">
           {tracks.map((track) => (
             <Track
               key={track.id}