fix(deps): update dependency mermaid to v11.15.0 [security], add e2e test (#37665)

Backport of #37662.

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This PR was written with the help of Claude Opus 4.7

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Co-authored-by: Giteabot <teabot@gitea.io>
Co-authored-by: Claude (Opus 4.7) <noreply@anthropic.com>

package.json

@@ -54,7 +54,7 @@
     "jquery": "4.0.0",
     "js-yaml": "4.1.1",
     "katex": "0.16.44",
-    "mermaid": "11.14.0",
+    "mermaid": "11.15.0",
     "online-3d-viewer": "0.18.0",
     "pdfobject": "2.3.1",
     "perfect-debounce": "2.1.0",