feat(contact): wire the form to a real endpoint at build time

The contact form could not be made to work by configuration. Two reasons,
both fixed here.

The endpoint never reached the browser. k8s/configmap.yaml carried
PUBLIC_CONTACT_FORM_ENDPOINT, but the Deployment has no env/envFrom pointing
at that ConfigMap, so it was dead config. Wiring it up would not have helped
either: this is a static Astro build, so import.meta.env.PUBLIC_* is inlined
when the image is built and the pod only serves pre-built HTML. Anything the
browser needs has to arrive as a build arg. Add ARG/ENV to the Dockerfile and
pass --build-arg from CI, sourced from a CONTACT_FORM_ENDPOINT Actions
secret. Rewrite the ConfigMap as documentation of what the build expects,
with an explicit warning not to add envFrom.

Submission behaviour was also wrong for visitors. A native POST would have
navigated them to the form provider's page; post in the background with
fetch() and send them to our own /thank-you/ instead. On failure the form now
stays put, re-enables the button and offers the phone and email fallback,
rather than silently losing the inquiry. Add a honeypot (_gotcha, which
Formspree treats as spam) positioned off-screen rather than display:none, so
bots that skip hidden fields still fill it.

Read data-endpoint at submit time rather than caching it at init, keeping the
attribute the single source of truth.

Tests: replace the old contact assertion, which faked a native GET
navigation, with one that mocks the endpoint and exercises the real fetch
path including the posted payload. Add a case for endpoint failure.

typecheck 0/0/0, build 0/0, seo:lint clean on 20 pages, e2e 44 passed.
The 4 remaining failures are the pre-existing stale nav assertions in
docs/launch-checklist.md section 2.1, untouched here.

Dockerfile

@@ -6,6 +6,14 @@ RUN npm ci
 
 FROM node:22-alpine AS build
 WORKDIR /app
+# Astro is a STATIC build: import.meta.env.PUBLIC_* is inlined into the HTML here, at image build
+# time. A runtime env var on the pod can never reach it (the pod only runs nginx over pre-built
+# files), so anything the browser needs must arrive as a build arg. CI passes these; see
+# .gitea/workflows/ci.yml.
+ARG PUBLIC_CONTACT_FORM_ENDPOINT=""
+ARG PUBLIC_SITE_URL="https://www.bluecapstrategies.com"
+ENV PUBLIC_CONTACT_FORM_ENDPOINT=$PUBLIC_CONTACT_FORM_ENDPOINT \
+    PUBLIC_SITE_URL=$PUBLIC_SITE_URL
 COPY --from=deps /app/node_modules ./node_modules
 COPY . .
 RUN npm run build