Add 'Reset demos' button to refresh canonical demo files

Existing accounts (including admin) seeded before new demos shipped
had no easy way to pull in the latest copies — the registration-time
seeder is intentionally non-destructive. The new badge action fetches
src/static/bundled-demos/manifest.json, confirms the overwrite, and
re-copies each canonical demo into code/. Open tabs of those files are
refreshed in place so the user sees the new content immediately.

src/static/bundled-demos/ ships the six canonical files plus the
manifest so this works in local mode and on a static-only host. The
Dockerfile now mirrors workspace/code/<demo>.py into bundled-demos/
during the image build, keeping the two locations in sync.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/static/index.html

@@ -118,6 +118,6 @@
     </div>
   </div>
 
-  <script type="module" src="/static/script.js?v=56"></script>
+  <script type="module" src="/static/script.js?v=57"></script>
 </body>
 </html>