Fix pending regen waveform segment boundary misalignment

_build_regen_pending_html was using segments[i+1][0] as each segment's
right edge, while _build_waveform_html uses the contact-edge midpoint
(segments[i][1] + segments[i+1][0]) / 2. Align both to the same formula
so segment boundaries don't shift when the pending placeholder is shown.

app.py

@@ -1995,16 +1995,19 @@ def _build_regen_pending_html(segments: list, regen_seg_idx: int, slot_id: str,
     active_color = "rgba(255,180,0,0.55)"
     duration = segments[-1][1] if segments else 1.0
 
+    # Compute contact edges: midpoint of overlap between consecutive segments.
+    # Matches the contact-edge formula used in _build_waveform_html's canvas JS
+    # so segment boundaries stay visually identical during pending regen.
+    contact_edges = []
+    for i in range(len(segments) - 1):
+        contact_edges.append((segments[i][1] + segments[i + 1][0]) / 2)
+
     seg_divs = ""
     for i, seg in enumerate(segments):
-        # Draw only the non-overlapping (unique) portion of each segment so that
-        # overlapping windows don't visually bleed into adjacent segments.
-        # Each segment owns the region from its own start up to the next segment's
-        # start (or its own end for the final segment).
-        seg_start = seg[0]
-        seg_end   = segments[i + 1][0] if i + 1 < len(segments) else seg[1]
-        left_pct  = seg_start / duration * 100
-        width_pct = (seg_end - seg_start) / duration * 100
+        left_t  = 0.0              if i == 0                    else contact_edges[i - 1]
+        right_t = duration         if i == len(segments) - 1    else contact_edges[i]
+        left_pct  = left_t  / duration * 100
+        width_pct = (right_t - left_t) / duration * 100
         color     = active_color if i == regen_seg_idx else seg_colors[i % len(seg_colors)]
         extra     = "border:2px solid #ffb300;animation:wf_pulse 0.8s ease-in-out infinite alternate;" if i == regen_seg_idx else ""
         seg_divs += (