fix horizontal seam: extend half-height by 2px for vertical overlap at mirror seam

Same approach as the horizontal overlap (55% width) — the half-canvas
now extends 2 pixels past the midline so top/bottom quadrants overlap
slightly, covering rounding/rasterization gaps.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/VisualizerWidget.cpp

@@ -390,7 +390,7 @@ void VisualizerWidget::render(QRhiCommandBuffer *cb) {
     m_lastBuildW = w;
     m_lastBuildH = h;
     if (m_mirrored) {
-      buildVertices(w * 0.55f, h / 2);
+      buildVertices(w * 0.55f, h / 2 + 2);
       buildCepstrumVertices(w, h);
     } else {
       buildVertices(w, h);