fix: focusable element when webcam expanded with no devices, add error test

- LaunchWindow: render sr-only <select> when webcamExpanded but
  cameraDevices.length === 0 (loading/error/empty), so keyboard users
  always have a focusable element even in no-camera states
- useCameraDevices.test: add error-branch test asserting error message,
  empty devices array and isLoading=false when enumerateDevices rejects

src/components/launch/LaunchWindow.tsx

@@ -365,7 +365,7 @@ export function LaunchWindow() {
 											/>
 										</>
 									))}
-								{!webcamExpanded && (
+								{(!webcamExpanded || cameraDevices.length === 0) && (
 									<select
 										value={webcamDeviceId || selectedCameraId}
 										onChange={(e) => {

src/hooks/useCameraDevices.test.ts

@@ -71,6 +71,19 @@ describe("useCameraDevices", () => {
 		expect(mockGetUserMedia).not.toHaveBeenCalled();
 	});
 
+	it("should set error state when enumeration fails", async () => {
+		mockEnumerateDevices.mockRejectedValueOnce(new Error("Permission denied"));
+
+		const { result } = renderHook(() => useCameraDevices(true));
+
+		await waitFor(() => {
+			expect(result.current.error).toBe("Permission denied");
+		});
+
+		expect(result.current.devices).toHaveLength(0);
+		expect(result.current.isLoading).toBe(false);
+	});
+
 	it("should fall back to first available device when selected device is unplugged", async () => {
 		const { result } = renderHook(() => useCameraDevices(true));