Game share-loop: instrument funnel, deep-link shares, /play metadata

Sharpen the existing daily-game share loop into something measurable (per Codex's
"instrument what you have, then feed people into it" plan), ahead of a Show HN launch.

Analytics:
- Per-game funnel events <game>_{arrival,started,completed,shared} (article_id=0).
  arrival = landed via a shared link (utm_source=game_share); started = first move
  (guess/find/flip); completed = solved/cleared/Full Bloom; shared = on share success.
- trackVisit() moved into the global layout so direct /play landings count; the
  server-rendered /a/ share page now creates a visitor token + sends a daily visit
  beacon (first-time /a/-only visitors were previously dropped).
- Admin "Games funnel" panel: arrivals / engaged / completed / shared, per game.

Sharing:
- Memory Match gains a Share button (it was the only game without one).
- All shares deep-link to the exact game+variant with a full https:// URL +
  utm_source=game_share (gameShareUrl helper), instead of a bare /play.
- "shared" is counted only after navigator.share()/clipboard.writeText() succeeds.

/play social metadata:
- /play served homepage canonical/OG (static SPA, ssr=false). postbuild script
  patches build/play.html's head to /play canonical/title/description/OG; fails the
  build if the homepage tags drift. Caddy try_files now serves {path}.html so /play
  is served from the patched file (snapshot in deploy/caddy/).

Tests: backend 352, frontend 27.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

goodnews/share.py

@@ -216,9 +216,14 @@ def render_share_page(article: dict, base_url: str, summary: str | None = None,
   <script>
 (function(){{
   try{{
-    var v=localStorage.getItem('goodnews:visitor')||'';
-    var b=JSON.stringify({{kind:'summary_viewed',article_id:{aid},visitor:v}});
-    if(navigator.sendBeacon) navigator.sendBeacon('/api/events', new Blob([b],{{type:'application/json'}}));
+    var v=localStorage.getItem('goodnews:visitor');
+    if(!v){{v=crypto.randomUUID?crypto.randomUUID():String(Math.random()).slice(2)+Date.now();localStorage.setItem('goodnews:visitor',v);}}
+    function beacon(o){{var b=JSON.stringify(o);if(navigator.sendBeacon)navigator.sendBeacon('/api/events',new Blob([b],{{type:'application/json'}}));}}
+    beacon({{kind:'summary_viewed',article_id:{aid},visitor:v}});
+    // This page is server-rendered (outside the Svelte app), so the SPA's daily
+    // visit isn't recorded for a /a/ landing — count it here, once per day per device.
+    var t=new Date().toISOString().slice(0,10);
+    if(localStorage.getItem('goodnews:visitday')!==t){{localStorage.setItem('goodnews:visitday',t);beacon({{kind:'visit',article_id:0,visitor:v}});}}
   }}catch(e){{}}
 }})();
 </script>