Fix broken header styling: stop gitignoring generated header.html

Tailwind v4's automatic content detection skips gitignored files, and
public/header.html was gitignored (generated) — so every header-only
utility class (hero-carousel, navbar-center, swap-rotate,
menu-horizontal, drop-shadow-2xl, etc.) got purged from the compiled
CSS on the last deploy. The header markup was correct, just entirely
unstyled.

Fixed two ways for certainty: added an explicit @source in each
site's style.css (Tailwind's documented way to force-include a
gitignored path), and also stopped gitignoring public/header.html
so automatic detection picks it up regardless — matching how the
original per-site repos worked before this generated a real file.
predev/prebuild still regenerate it on every dev/build, so it can't
go stale.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SBzcNkW7JcYipnAX6HfgmK

sites/scouting/.gitignore

@@ -4,5 +4,3 @@ dist/
 .env.local
 .claude/settings.local.json
 
-# generated from shared/header.template.html — see scripts/render-header.mjs
-public/header.html