fix: use same-origin banner proxy for project og:image (composer thumbnail)
Homepage link cards used /og-hero.png (~320KB, atmosphereaccount.com).
Project pages used full-size cdn.bsky.app banner URLs (~650KB+), which
matched Cardyb extract but often showed text-only cards in the Bluesky
composer. Use absolute /api/registry/banner/{did} for og/twitter image
metadata so embeds follow the same origin/size pattern as the homepage.
Remove unused bsky CDN banner helpers from lib/avatar.ts.
Made-with: Cursor