Suppress Mermaid render error artifacts

Configure Mermaid to suppress its built-in error renderer in both
interactive UI previews and generated document HTML. Render failures now
surface through the product toast flow with a safe inline fallback instead
of leaving raw Mermaid diagnostics in the document body.

Signed-off-by: Cursor Agent <cursoragent@cursor.com>

Co-authored-by: Émile Ré <nemile.re@gmail.com>

pkg/docgen/template.html

@@ -374,7 +374,7 @@
     <script>{{.MermaidJS}}</script>
     <script>
         window.__mermaidReady = false;
-        mermaid.initialize({ startOnLoad: false, theme: 'neutral' });
+        mermaid.initialize({ startOnLoad: false, theme: 'neutral', suppressErrorRendering: true });
         document.addEventListener('DOMContentLoaded', function () {
             mermaid.run().then(function () {
                 window.__mermaidReady = true;