fix: clip section sub-track content to section bounds

When section duration is resized shorter, sub-track clips and MIDI
notes were still drawn outside the section's visual rectangle.
Added ctx.save/clip/restore around sub-track rendering.

app/static/js/app.jsx

@@ -712,6 +712,10 @@ const WaveformLane = ({
       const subTrackCount = Math.min(subTracks.length, 4);
       const subTrackHeight = (height - 20) / Math.max(1, subTrackCount);
       const subColors = ['#fbbf24', '#a78bfa', '#ec4899', '#10b981'];
+      ctx.save();
+      ctx.beginPath();
+      ctx.rect(secStartLocal, 2, secWidth, height - 4);
+      ctx.clip();
       for (let stIdx = 0; stIdx < subTrackCount; stIdx++) {
         const sub = subTracks[stIdx];
         if (!sub) continue;
@@ -767,6 +771,7 @@ const WaveformLane = ({
           });
         });
       }
+      ctx.restore();
     });
 
     // Draw MIDI items