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.

backend/src/routes/nodes.ts

@@ -37,7 +37,7 @@ function mintPilotEnrollment(nodeId: number, req: Request): { token: string; exp
   const forwardedProto = req.headers['x-forwarded-proto'];
   const protoHeader = Array.isArray(forwardedProto) ? forwardedProto[0] : forwardedProto;
   const protocol = protoHeader || req.protocol || 'http';
-  const host = req.get('host') || 'localhost:3000';
+  const host = req.get('host') || 'localhost:1852';
   const primaryUrl = `${protocol}://${host}`;
 
   const dockerRun =