fix: correct crossfade indicator position on waveform

Overlap zone is [seg[i+1].start, seg[i].end] — both halves occupy the
same time range, so the hatch must be drawn there (end of seg i), not
at the start of seg i+1. Previously appeared only 'after c' because
overlapStart was used instead of the correct seg[i].end boundary.

app.py

@@ -1750,11 +1750,16 @@ def _build_waveform_html(audio_path: str, segments: list, slot_id: str,
     }});
 
     // ── Crossfade overlap indicators ──
+    // The overlap region spans [segments[i+1][0], segments[i][1]]:
+    //   - last crossfadeSec of seg i fades OUT
+    //   - first crossfadeSec of seg i+1 fades IN
+    // Both halves are the same duration (crossfadeSec) and occupy the same
+    // time range, so we draw the hatch centred on the join boundary.
     if (crossfadeSec > 0 && segments.length > 1) {{
       for (let i = 0; i < segments.length - 1; i++) {{
-        // Overlap zone: where segment i ends and segment i+1 starts
-        const overlapStart = segments[i+1][0];
-        const overlapEnd   = Math.min(segments[i][1], overlapStart + crossfadeSec);
+        // The actual overlap window in time
+        const overlapStart = segments[i+1][0];       // = seg_i.end - crossfadeSec
+        const overlapEnd   = segments[i][1];          // = seg_i.end
         const xL = (overlapStart / duration) * W;
         const xR = (overlapEnd   / duration) * W;
         // Diagonal hatch pattern over the overlap zone
@@ -1772,12 +1777,14 @@ def _build_waveform_html(audio_path: str, segments: list, slot_id: str,
           ctx.stroke();
         }}
         ctx.restore();
-        // Label
-        ctx.fillStyle = 'rgba(255,255,255,0.5)';
-        ctx.font = '9px sans-serif';
+        // Label — centred on the join boundary line, straddling both segments
+        ctx.fillStyle = 'rgba(255,255,255,0.6)';
+        ctx.font = 'bold 9px sans-serif';
         const cfW = xR - xL;
-        if (cfW > 20) {{
-          ctx.fillText('CF', xL + (cfW - ctx.measureText('CF').width) / 2, H - 3);
+        const label = '\u21C4 CF';   // ⇄ CF
+        const lw = ctx.measureText(label).width;
+        if (cfW > lw + 4) {{
+          ctx.fillText(label, xL + (cfW - lw) / 2, H - 3);
         }}
       }}
     }}