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/assets/site.css

@@ -376,6 +376,8 @@ textarea{resize:vertical;font:inherit}
 /* ── bio header (wall/profile page) ── */
 .bio-head{display:flex;gap:22px;align-items:center;flex-wrap:wrap;padding:8px 0 4px}
 .bio-avatar{width:96px;height:96px;border-radius:50%;object-fit:cover;border:2px solid var(--mint);flex:0 0 auto}
+.bio-badge{width:74px;height:74px;border-radius:12px;object-fit:cover;flex:0 0 auto;border:1px solid var(--line-strong)}
+.btn.disabled{opacity:.45;pointer-events:none;filter:grayscale(.3)}
 .bio-meta{flex:1;min-width:220px}
 .bio-socials{display:flex;gap:8px;flex-wrap:wrap;margin-top:12px}
 .bio-socials a{font-size:12px;font-weight:700;color:var(--mint);border:1px solid var(--line-strong);