Add a custom social media preview image to the repository #856

Summary

The nostream repo has no custom social media preview image — GitHub renders the default (repo name on a plain background) when the repo is shared on Slack, X, Discord, Telegram, etc. Upload a custom one so link shares carry the project's branding.

Spec

• Settings → General → Social preview → Upload an image
• Size: at least 640×320 px; 1280×640 px (2:1) recommended by GitHub for best display
• Content, consistent with the site's design language (--bg: #08080a dark ground, --pink: #ff2e88 accent):
  • The nostream logo/mark (note: Recolor the nostream icon from purple to the site's pink/magenta accent #853 proposes recoloring it purple → pink; if that lands first, generate from the pink version)
  • The wordmark "nostream" and the tagline ("your relay, your rules")
• Source it from the hero-animation design assets (website/hero-animation/, src/assets/logo.png) so it stays coherent with the site and can be regenerated, not a one-off export
• Note the difference from Add a social share preview (OG image + complete meta) #851: that one is the website's OG image (served by ema.tnsor.network via meta tags); this ticket is the GitHub repository's social preview (uploaded in repo settings, used when the repo URL itself is shared). They can share a source image but are distinct artifacts.

Acceptance

• 1280×640 image generated from the brand assets
• Uploaded in repo settings; sharing https://github.com/cameri/nostream on Slack/X/Discord shows the card