fix OG tags: remove layout defaults, use OG endpoint URL
- remove default OG meta tags from layout (they override page-specific ones)
- point og:image to our /og endpoint instead of raw PDS blob URL
- add OG meta tags to homepage
- crawlers now get consistent, proxied images from our endpoint
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>