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/robots.txt

@@ -0,0 +1,5 @@
+User-agent: *
+Allow: /
+Disallow: /my
+Disallow: /api/
+Sitemap: https://instantadpay.com/sitemap.xml