fix(library): generate every post cover from one template

Three posts shipped an `ogImage` pointing at a file that was never
committed, so the library index rendered broken images and their
`og:image`, JSON-LD, and sitemap entries all 404'd. Several others were
authored without the brand font loaded or with the title clipping off the
bottom edge.

Covers were hand-made per post with no generator, which is why they
drifted. Adds `bun run library:covers`, rendering each cover from the
post's frontmatter title using the reference template already encoded in
the docs OG route, and regenerates all 20 so the grid is uniform.

Line widths come from the font's real advance metrics rather than an
average-glyph-width estimate: the template joins words with non-breaking
spaces to dodge a Satori space-measurement bug, which leaves hyphens as
the only fallback break points, so an under-measured line breaks
mid-compound.

Also drops six orphaned `cover.png` sources left over from the JPEG
compression pass in #5528.

Author: waleedlatif1