Browser extension: detect and subscribe to standard.site publications on ATProto

feat: add an updates blog with /posts pages and an atom feed master

Markdown posts in web/src/content/posts render at /posts/<slug>, list at /posts, and preview as a short feed on the homepage. A hand-written atom feed at /atom.xml is autodiscovered from every page. The layout renders a rel="site.standard.document" link when a post's frontmatter carries an atUri, and trailingSlash is pinned to "never" so canonicals match the addresses document records will claim. The wordmark is now inlined via a ?raw import: the old readFileSync against import.meta.url broke under any non-default --outDir.