Badge emblems, profile social links, and SEO/OG social previews

- Achievement badge share-image now draws a milestone emblem (⚡🎯⭐🏆) in the medal
- Profile: Facebook/X/YouTube/Instagram/TikTok/Telegram/LinkedIn/Website links, shown on the public bio page
- OG + Twitter Card tags on homepage/ledger/contract (hero banner as share image)
- Per-member OG tags server-injected into /wall/<username> so shared bio links preview with name/bio/avatar
- robots.txt + sitemap.xml; .txt/.xml MIME types

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

public/assets/site.css

@@ -373,6 +373,10 @@ textarea{resize:vertical;font:inherit}
 .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-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);
+  border-radius:999px;padding:4px 12px;text-decoration:none}
+.bio-socials a:hover{border-color:var(--mint)}
 .bio-qr{flex:0 0 auto;background:var(--panel);border:1px solid var(--line-strong);border-radius:14px;padding:10px}
 .bio-qr img{display:block;border-radius:8px;background:#eef7f3}
 /* ── wall page ── */