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.

src/styles/global.css

@@ -1078,6 +1078,27 @@ textarea:focus {
   font-size: 0.95rem;
 }
 
+/* Honeypot: kept in the layout but out of sight and out of the tab order. Not display:none —
+   some bots skip hidden fields, which would defeat the point. */
+.gotcha {
+  position: absolute;
+  left: -9999px;
+  width: 1px;
+  height: 1px;
+  overflow: hidden;
+}
+
+.form-status {
+  margin: 0;
+  font-size: 0.95rem;
+  font-weight: 700;
+  color: var(--muted);
+}
+
+.form-status.is-error {
+  color: #b3261e;
+}
+
 .site-footer {
   background: #10293d;
   color: rgba(255, 255, 255, 0.78);