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/track.js

@@ -0,0 +1,18 @@
+(function(){
+  function getSource(){
+    try{
+      let s=sessionStorage.getItem('ctb.src');
+      if(s)return s;
+      const p=new URLSearchParams(location.search);
+      s=p.get('utm_source')||p.get('src')||'';
+      if(!s&&document.referrer){try{const h=new URL(document.referrer).hostname;if(h&&h!==location.hostname)s=h}catch(e){}}
+      s=s||'(direct)';
+      sessionStorage.setItem('ctb.src',s);
+      return s;
+    }catch(e){return '(direct)'}
+  }
+  window.ctbGetSource=getSource;
+  const path=location.pathname.replace(/\/$/,'')||'/';
+  const page=path==='/'?'bridge':(path==='/start'?'start':null);
+  if(page)fetch('/api/public/track',{method:'POST',headers:{'Content-Type':'application/json'},body:JSON.stringify({event:page,source:getSource()})}).catch(()=>{});
+})();