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>

frontend/src/routes/+page.svelte

@@ -14,7 +14,7 @@
   import { auth, refresh as refreshAuth, isFollowing, toggleFollow, followKeys } from '$lib/auth.svelte.js';
   import { prefs, initPrefs, active as prefsActive, applyPrefAction, persistPrefs, syncPrefsOnLogin } from '$lib/prefs.svelte.js';
   import { initHistory, deviceIds, record, loadServerHistory } from '$lib/history.svelte.js';
-  import { trackVisit, track } from '$lib/analytics.js';
+  import { track } from '$lib/analytics.js';
   import { pwa, installApp, dismissPwa } from '$lib/pwa.svelte.js';
   import { ritualState, markBriefSeen } from '$lib/ritual.js';
 
@@ -488,7 +488,7 @@
     seenIds = new Set(P.loadJSON(SEEN_KEY, []));
     dismissed = new Set(P.loadJSON(DISMISSED_KEY, []));
     refreshAuth();
-    trackVisit();
+    // trackVisit() now fires once in the global layout (covers every landing page).
     if (selected === 'search') { searchText = searchQuery; searchOpen = true; }  // prefill on direct/shared link
     // Instant paint: render the last saved Today brief immediately and refresh
     // it behind the scenes, so the first view never blocks on a (personalized,