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>

web/standalone/.env.example

@@ -42,11 +42,11 @@ VITE_WASM_ROOT=/wasm
 # can't follow redirects.
 # VITE_WAITLIST_URL=https://www.pcbjam.com/api/waitlist
 
-# Plausible analytics. Unset ⇒ no tracking (dev/checkout default). Set the
-# data-domain to enable; override the script URL only if the default plausible.io
-# script won't load under COEP require-corp (then self-host/proxy on cdn.pcbjam.com).
-# VITE_PLAUSIBLE_DOMAIN=demo.pcbjam.com
-# VITE_PLAUSIBLE_SRC=https://plausible.io/js/script.js
+# Plausible analytics. Unset ⇒ no tracking (dev/checkout default). Set the shared
+# site's pa-*.js script URL to enable (the pa- id keys the dashboard, no
+# data-domain). Point it at a self-hosted/proxied copy on cdn.pcbjam.com if the
+# plausible.io script won't load under COEP require-corp.
+# VITE_PLAUSIBLE_SRC=https://plausible.io/js/pa-KjNS9YmidydULZTstsjRg.js
 
 # Where a backend project's DOCUMENT content lives ("api" default | "ydoc").
 # Same /p/<project> URLs either way. "api": file bytes come from the REST