fix(capture): derive video ids from pipelines and guard empty list (#711)

when NEKO_CAPTURE_VIDEO_PIPELINES is set without capture.video.ids the
VideoIDs list stayed empty, so capture.Video().IDs() returned [] and the
first websocket signal request crashed the whole server with
"index out of range [0] with length 0" in signalRequest.

now we fill VideoIDs from the configured pipeline keys (sorted by id)
whenever no ids are set explicitly, and signalRequest returns a plain
error instead of panicking when there is no video stream at all.

added tests for the config loader, the capture manager and the signal
handler so this path stays covered.

server/internal/websocket/handler/signal.go

@@ -30,6 +30,9 @@ func (h *MessageHandlerCtx) signalRequest(session types.Session, payload *messag
 	// use default first video, if not provided
 	if video.Selector == nil {
 		videos := h.capture.Video().IDs()
+		if len(videos) == 0 {
+			return errors.New("no video streams available")
+		}
 		video.Selector = &types.StreamSelector{
 			ID:   videos[0],
 			Type: types.StreamSelectorTypeExact,