feat(06-02): add segment overlay canvas rendering and click interaction

- Create lightsync/frontend/timeline/segments.js with SegmentOverlay class
- Integrate SegmentOverlay into timeline.js render pipeline (between row banding and waveform)
- Add loadSegments method to TimelineCanvas
- Wire loadSegments call in app.js loadedmetadata handler

lightsync/frontend/app.js

@@ -492,6 +492,8 @@ function initTimeline() {
         const path = `/app/shows/${filename}`;
         timeline.audioDuration = audio.duration;
         timeline.loadWaveform(path);
+        // Load structural segments for overlay
+        timeline.loadSegments(path);
         // Load beats and update BPM display when analysis completes
         timeline.loadBeats(path).then(() => {
             if (timeline.beatGrid.tempoBpm) {