noagendastream.sh: fix syntax error and player detection

Replace the invalid 'else [ ... ];then' with elif, detect players with
command -v, and print an error when no player is installed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

noagendastream.sh

@@ -1,10 +1,14 @@
 #!/bin/bash
 
+STREAM="http://listen.noagendastream.com/noagenda"
 
-if [ -f /usr/bin/mpv ];then
-	mpv "http://listen.noagendastream.com/noagenda"
-elif [ -f "/usr/bin/vlc" ];then
-    vlc "http://listen.noagendastream.com/noagenda"
-else [ -f "/usr/bin/xdg-open" ];then
-    xdg-open "http://listen.noagendastream.com/noagenda"
+if command -v mpv >/dev/null 2>&1; then
+	mpv "$STREAM"
+elif command -v vlc >/dev/null 2>&1; then
+	vlc "$STREAM"
+elif command -v xdg-open >/dev/null 2>&1; then
+	xdg-open "$STREAM"
+else
+	echo "No player found. Install mpv or vlc." >&2
+	exit 1
 fi