site: entry, stack and freshness pages have a thinner <head> than the home page (no theme-color, feed link, og:image size) #104

Why it matters
The home page emits theme-color for both colour schemes, an Atom rel="alternate" link, and og:image:width/height/alt. The other 420 pages do not, so the browser chrome flashes the wrong colour on dark-mode phones, feed readers cannot autodiscover the feed from an entry link, and some social scrapers fall back to a small card. This is the same duplicated-head problem described in the fonts issue; fixing one should fix both.

Evidence

• Home: site/build.mjs:270-296.
• Entry: site/lib/entry-page.mjs:93-98, 114-125. Stack: site/lib/stack-page.mjs:94, 120-134. Freshness: site/lib/freshness.mjs:~100-110.

Suggested approach
Factor the shared lines of headMetaHtml into a helper in site/lib/shared.mjs taking {title, description, url, image, type} and call it from all four page builders. Add a lib.test.mjs case that every page type's head contains the same baseline tags.

Done when
All page types carry the same baseline meta set and a test enforces it.