Wall page: mint QR, achievement badge, admin-ad backfill, join gated on views

- QR is mint-green (still high-contrast for reliable scanning).
- The wall shows the owner's achievement badge next to their avatar.
- Positions 2 & 3 fill with upline line-banners, then ADMIN ADS when there's no
  upline (configurable via data/admin-wall-ads.json; sensible default) so the
  wall is never sparse.
- "Join free through this wall" is disabled until the visitor has viewed every
  ad on the wall, with a "X/Y viewed" prompt.

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

public/view.html

@@ -4,7 +4,7 @@
 <meta charset="utf-8">
 <meta name="viewport" content="width=device-width,initial-scale=1">
 <title>Viewing ad — InstantAdPay</title>
-<link rel="stylesheet" href="/assets/site.css?v=20260906m">
+<link rel="stylesheet" href="/assets/site.css?v=20260907s">
 <style>
   html,body{height:100%;margin:0;overflow:hidden}
   .vw{display:flex;flex-direction:column;height:100vh;height:100dvh;background:var(--bg,#04110c);color:var(--ink,#e8fff7)}