Derive mermaid CDN version from package.json; bump to 11.16.0

The mermaid version was pinned in two places that had to be kept in
sync by hand: package.json (a Dependabot tracking shim, since there is
no Node build) and a hardcoded CDN import URL in head_end.html, which is
the version the site actually loads. Dependabot only edits package.json,
so its bump PRs left the CDN URL untouched and the two pins drifted.

Make package.json the single source of truth: symlink it into _data/ so
Jekyll loads it as site.data.package, and derive the CDN version via
Liquid in head_end.html. Future Dependabot bumps now propagate to the
live version automatically.

Also bumps mermaid 11.15.0 -> 11.16.0, which exercises the new
mechanism. Verified with jekyll build: rendered pages import
mermaid@11.16.0 and the project sequence diagram renders cleanly.

Supersedes #367. Closes #370.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Signed-off-by: Michael Harp <mike@mikeharp.com>

Author: miharp

_data/package.json

@@ -0,0 +1 @@
+../package.json

_includes/jekyll_vitepress/head_end.html

@@ -12,7 +12,7 @@
 </script>
 
 <script type="module">
-  import mermaid from 'https://cdn.jsdelivr.net/npm/mermaid@11.15.0/dist/mermaid.esm.min.mjs';
+  import mermaid from 'https://cdn.jsdelivr.net/npm/mermaid@{{ site.data.package.dependencies.mermaid }}/dist/mermaid.esm.min.mjs';
   mermaid.initialize({ startOnLoad: false });
 
   function renderMermaid() {

package.json

@@ -2,6 +2,6 @@
   "name": "openvox-docs",
   "private": true,
   "dependencies": {
-    "mermaid": "11.15.0"
+    "mermaid": "11.16.0"
   }
 }