feat: add combined docker+API service widgets for gitea/coolify/immich/n8n/umami/headscale

One new `service` widget type (nested discriminated union on `service`)
rather than six, so a single config entry shows both docker container
health and a service-specific stat (repo count, project list, photo
count, workflow count, active users, users/nodes) - avoiding the
overhead of configuring a docker widget and a separate service widget
per container. All six collectors hit the service's container name +
internal port directly on falcon_network, the same container-to-
container pattern just proven out for Traefik's own API, avoiding
vpn-only/hairpin-NAT entirely.

Also adds the public/private config split that was scoped in the
original project plan but never built: lib/config/public.ts strips
apiToken/apiKey/password fields before the config reaches the browser
via SSR or the WS config topic - required before any widget could
carry a real secret. Verified via a throwaway secret field that it's
absent from both the SSR HTML and the WS config:update frame.

Endpoint shapes verified live against the running gitea/coolify/immich/
n8n/umami/headscale containers before committing (unauthenticated
requests correctly 401/200 on every target route; gitea's
X-Total-Count header confirmed present).

lib/collectors/services/headscale.ts

@@ -0,0 +1,30 @@
+import type { HeadscaleServiceWidget } from "@/lib/config/schema";
+import type { ServiceStat } from "@/lib/types/service-result";
+import { serviceBaseUrl } from "./base-url";
+
+async function fetchList(url: string, apiToken: string, label: string, key: "users" | "nodes"): Promise<unknown[]> {
+  const response = await fetch(url, {
+    headers: { Authorization: `Bearer ${apiToken}` },
+    signal: AbortSignal.timeout(5_000),
+  });
+  if (!response.ok) {
+    throw new Error(`Headscale ${label} API returned ${response.status}`);
+  }
+  const body = (await response.json()) as Record<string, unknown[]>;
+  return body[key] ?? [];
+}
+
+export async function collectHeadscale(widget: HeadscaleServiceWidget): Promise<{ stats: ServiceStat[] }> {
+  const base = serviceBaseUrl(widget);
+  const [users, nodes] = await Promise.all([
+    fetchList(`${base}/api/v1/user`, widget.apiToken, "user", "users"),
+    fetchList(`${base}/api/v1/node`, widget.apiToken, "node", "nodes"),
+  ]);
+
+  return {
+    stats: [
+      { label: "Users", value: String(users.length) },
+      { label: "Nodes", value: String(nodes.length) },
+    ],
+  };
+}