fix: move segment color boundary to crossfade midpoint

Color edge was at seg[i+1].start (crossfade begins) but should be at
seg[i+1].start + crossfade/2 — the 50/50 blend point, which matches
where the cut would be with no crossfade and where listeners perceive
the transition.

app.py

@@ -1711,11 +1711,12 @@ def _build_waveform_html(audio_path: str, segments: list, slot_id: str,
     ctx.fillRect(0, 0, W, H);
 
     segments.forEach(function(seg, idx) {{
-      // Draw only each segment's unique (non-overlapping) region:
-      // from its start up to the next segment's start, or its own end if last.
+      // Color boundary = midpoint of the crossfade zone = where the blend is
+      // 50/50. This is also where the cut would land if crossfade were 0, and
+      // where the listener perceptually hears the transition to the next segment.
       const x1 = (seg[0] / duration) * W;
       const xEnd = idx + 1 < segments.length
-        ? (segments[idx + 1][0] / duration) * W
+        ? ((segments[idx + 1][0] + crossfadeSec / 2) / duration) * W
         : (seg[1] / duration) * W;
       ctx.fillStyle = segColors[idx % segColors.length];
       ctx.fillRect(x1, 0, xEnd - x1, H);