fix: Mermaid fullscreen — fill viewport instead of showing tiny centered diagram

frontend/js/mermaid-viewer.js

@@ -205,10 +205,6 @@ function openFullscreen(svgWrapper) {
   content.innerHTML = svgWrapper.innerHTML;
   var svg = content.querySelector('svg');
   if (svg) {
-    svg.style.width = '100%';
-    svg.style.height = '100%';
-    svg.style.maxWidth = '95vw';
-    svg.style.maxHeight = '95vh';
     svg.style.cursor = 'default';
   }
 

frontend/style.css

@@ -7676,7 +7676,14 @@ body.popup-mode .content-area {
   z-index: 1;
 }
 .mermaid-fullscreen-close:hover { background: rgba(255,255,255,0.1); }
-.mermaid-fullscreen-content { max-width: 95vw; max-height: 95vh; }
+.mermaid-fullscreen-content {
+  width: 95vw; height: 95vh;
+  display: flex; align-items: center; justify-content: center;
+}
+.mermaid-fullscreen-content svg {
+  width: 100%; height: 100%;
+  max-width: 100%; max-height: 100%;
+}
 
 .mermaid-focus-panel {
   position: fixed; top: 0; right: 0; bottom: 0; width: 480px;