fix(headroom): support Docker sidecar proxy

Treat configured Headroom proxy as running when its /health endpoint
responds, even if local headroom CLI is not installed. Dashboard
Start/Stop stays limited to local loopback proxies while external
Docker sidecars can be enabled via HEADROOM_URL.

Closes #1948

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/unit/headroom-detect.test.js

@@ -0,0 +1,36 @@
+import { describe, it, expect, vi, afterEach } from "vitest";
+
+const mocks = vi.hoisted(() => ({
+  execSync: vi.fn(() => { throw new Error("not found"); }),
+}));
+
+vi.mock("child_process", () => ({
+  execSync: mocks.execSync,
+}));
+
+import { getHeadroomStatus, isLoopbackHeadroomUrl } from "../../src/lib/headroom/detect.js";
+
+afterEach(() => {
+  vi.clearAllMocks();
+});
+
+describe("headroom detect", () => {
+  it("treats a reachable external proxy as running without local CLI", async () => {
+    global.fetch = vi.fn(async () => new Response("ok", { status: 200 }));
+
+    const status = await getHeadroomStatus("http://headroom:8787");
+
+    expect(status.installed).toBe(false);
+    expect(status.running).toBe(true);
+    expect(status.localUrl).toBe(false);
+    expect(status.canStart).toBe(false);
+    expect(global.fetch).toHaveBeenCalledWith("http://headroom:8787/health", expect.any(Object));
+  });
+
+  it("recognizes loopback URLs for managed local mode", () => {
+    expect(isLoopbackHeadroomUrl("http://localhost:8787")).toBe(true);
+    expect(isLoopbackHeadroomUrl("http://127.0.0.1:8787")).toBe(true);
+    expect(isLoopbackHeadroomUrl("http://headroom:8787")).toBe(false);
+    expect(isLoopbackHeadroomUrl("not-a-url")).toBe(false);
+  });
+});