analytics: consolidate everything onto one shared Plausible pa- script

All pcbjam properties now load the same new-format Plausible script
(https://plausible.io/js/pa-KjNS9YmidydULZTstsjRg.js — one dashboard,
segment by hostname) instead of per-domain data-domain sites:

- site: replace Vercel Analytics with the pa- snippet in BaseLayout
  (is:inline, prod-only, crossorigin="anonymous" for the COEP
  require-corp routes in vercel.json); drop @vercel/analytics dep.
  Legal pages (cookies/privacy) rewritten Vercel -> Plausible.
- standalone: initAnalytics() rewritten for the new script format
  (queue shim + plausible.init(), async, no data-domain). The gate is
  now VITE_PLAUSIBLE_SRC (script URL, deploy-time opt-in) — unset means
  no tracking, so dev checkouts and third-party GPL builds stay clean.
  VITE_PLAUSIBLE_DOMAIN is gone.
- deploy: build-demo/build-editor --plausible now takes the script URL;
  release.yml/deploy-demo.yml hardcode the (public) pa- URL, so the
  repo Actions variable PLAUSIBLE_DOMAIN is no longer used.

Verified: standalone tsc + vite build (pa- URL present with env, absent
without, crossOrigin kept); astro build has the snippet on all layout
pages and no vercel/insights references; astro dev stays untracked.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

site/package.json

@@ -17,7 +17,6 @@
     "@astrojs/mdx": "^6.0.3",
     "@astrojs/vercel": "^10.0.8",
     "@fontsource/oswald": "^5.2.8",
-    "@vercel/analytics": "^2.0.1",
     "astro": "^6.4.4",
     "resend": "^6.12.4"
   },