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) => {