fix: add matrix mod header spacer

Add .mmod-head spacer to header row to align with voice row's
sticky-right .mmod block, matching .minst-head pattern on left.
Width: 192px (64+64+56 children widths + 4px padding-left).

web_realart/public/control/control.css

@@ -134,6 +134,7 @@ button.queued { animation: queued-blink 600ms ease-in-out infinite alternate; }
   background: #1a1a1a; color: #ccc; border: 1px solid #333; flex-shrink: 0; }
 .kit-btn { flex: 1; padding: 10px 4px; font-size: 11px; }
 .minst-head { width: 78px; flex-shrink: 0; }
+.mmod-head { width: 192px; flex-shrink: 0; }
 .mmod { position: sticky; right: 0; z-index: 2; background: #111;
   display: flex; gap: 2px; align-items: center; flex-shrink: 0; padding-left: 4px; }
 .mmod-src, .mmod-tgt { width: 64px; font-size: 9px; background: #1a1a1a; color: #ccc; border: 1px solid #333; }

web_realart/public/control/control.js

@@ -320,6 +320,9 @@ function renderMatrix() {
     num.textContent = (bar % 4 === 0) ? String(bar + 1) : "";
     headerRow.appendChild(num);
   }
+  const modHead = document.createElement("span");
+  modHead.className = "mmod-head";
+  headerRow.appendChild(modHead);
   container.appendChild(headerRow);
 
   // Voice rows