Add simple traffic analytics with source attribution and conversion funnel

- track.js captures first-touch source per session (utm_source/src param
  or external referrer domain) and logs bridge/start page views.
- Join clicks now carry the source; all events aggregate per-source in
  data/analytics.json on the persistent volume.
- Admin: new Traffic & Conversions card with funnel totals (bridge ->
  start -> join click rates) and a per-source breakdown table.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

public/start.html

@@ -16,4 +16,4 @@
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-<script src="/start.js"></script></body></html>
+<script src="/track.js"></script><script src="/start.js"></script></body></html>