fix: ensure consistent timeline-waveform alignment with always-visible scrollbar

Changed waveforms container from overflow-y-auto to overflow-y-scroll to ensure
the vertical scrollbar is always visible. This maintains consistent width alignment
between the timeline and waveform areas, preventing playhead marker misalignment
that occurred when scrollbar appeared/disappeared based on track count.

Fixes the issue where timeline was slightly wider than waveforms when vertical
scrollbar was present, causing ~15px offset in playhead position.

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

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

components/tracks/TrackList.tsx

@@ -816,7 +816,7 @@ export function TrackList({
         <div
           ref={waveformScrollRef}
           onScroll={handleWaveformScroll}
-          className="flex-1 overflow-y-auto overflow-x-hidden custom-scrollbar"
+          className="flex-1 overflow-y-scroll overflow-x-hidden custom-scrollbar"
         >
           <div className="flex flex-col">
             {tracks.map((track) => (