feat: change default listen port from 3000 to 1852 (#756)

Updates the backend listen port, Vite dev proxy target, Docker EXPOSE,
compose port mapping, .env.example default, GitHub Actions smoke-test
default, healthcheck URLs, and every doc/example reference. Test fixtures
that include example URLs were updated for consistency, though their
assertions are port-agnostic.

The rate-limit value of 3000 in middleware/rateLimiters.ts and the
3000 entry in WEB_UI_PORTS (which detects user containers like Grafana)
are intentionally untouched.

.github/actions/start-app/action.yml

@@ -13,7 +13,7 @@ inputs:
     default: '/tmp/compose'
   port:
     required: false
-    default: '3000'
+    default: '1852'
   skip-backend-build:
     description: >
       When 'true', skip running `npm run build` in backend/ and assume the