devlog: fix OG metatags

OG image URLs in both the post page and the index were relative (/api/og?...), which crawlers can't resolve. Prefixed with PUBLIC_SITE_URL throughout.

The index page had no OG image at all and was using twitter:card: summary (small image). Now has a proper image and summary_large_image to match the post pages.

Also removed a duplicate pair of og:image:width / og:image:height tags on the post page.