fix: pick a free port for the desktop backend when 30001 is taken (#1663)

src/ui/tests/lib/local-backend-port.test.ts

@@ -0,0 +1,25 @@
+import { afterEach, describe, expect, it } from "vitest";
+import { localBackendPort } from "@/lib/electron";
+
+function setElectronAPI(api: unknown) {
+  (window as unknown as { electronAPI?: unknown }).electronAPI = api;
+}
+
+afterEach(() => {
+  delete (window as unknown as { electronAPI?: unknown }).electronAPI;
+});
+
+describe("localBackendPort", () => {
+  it("uses the port the desktop app picked", () => {
+    setElectronAPI({ backendPort: 41234 });
+    expect(localBackendPort()).toBe(41234);
+  });
+
+  it("falls back to 30001", () => {
+    expect(localBackendPort()).toBe(30001);
+    setElectronAPI({ backendPort: Number.NaN });
+    expect(localBackendPort()).toBe(30001);
+    setElectronAPI({ backendPort: 70000 });
+    expect(localBackendPort()).toBe(30001);
+  });
+});