Dashboard UI: subdomain checkbox, drop proxy path/host

Follow the backend to the subdomain-only model:
- Service editor + create form: replace the Proxy path / Proxy host text fields
  with a single "Expose" checkbox (writes proxy: { caddy: {} } or removes proxy).
- ServiceSummary/DeploymentSummary type: proxy_path/proxy_host → subdomain.
- ServiceCard / ServiceDetail: show the subdomain, linking to
  <subdomain>.<domain> via a new subdomainUrl() helper (domain derived from the
  dashboard's own host; null on a bare host / off mode).

app/src/lib/labels.ts

@@ -47,3 +47,16 @@ export function behaviorLabel(behavior: string): string {
 export function stackLabel(stack: string): string {
   return STACK_LABELS[stack] ?? stack
 }
+
+/**
+ * Full URL for a service exposed at <subdomain>.<gateway.domain>. The domain is
+ * derived from the dashboard's own host (it is served at castle-app.<domain>), so
+ * this returns null when the dashboard is on a bare host (off mode, no subdomains).
+ */
+export function subdomainUrl(subdomain: string): string | null {
+  if (typeof window === "undefined") return null
+  const { protocol, hostname } = window.location
+  const labels = hostname.split(".")
+  if (labels.length <= 2) return null
+  return `${protocol}//${subdomain}.${labels.slice(1).join(".")}`
+}