diagrams: force light theme for mermaid rendering

Text and connectors were invisible because mmdc rendered with a dark
theme (white text/edges) on the white diagram background. Explicitly
pass -t default to force dark text on light background.

gui/chatblockmodel.cpp

@@ -966,7 +966,8 @@ QString ChatBlockModel::diagramUrl(const QString &source, const QString &languag
         proc.start(mmdc, {
             QStringLiteral("-i"), input.fileName(),
             QStringLiteral("-o"), svgPath,
-            QStringLiteral("-b"), QStringLiteral("transparent")
+            QStringLiteral("-t"), QStringLiteral("default"),
+            QStringLiteral("-b"), QStringLiteral("white")
         });
 
         if (!proc.waitForFinished(15000))