fix: align cmd_stop else branch with _play_loop finally cleanup

The else branch (no active task) only cleared current, task, and
duck_vol. Now resets all play state fields to match _play_loop's
finally block: done_event, duck_task, progress, and cur_seek.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

plugins/music.py

@@ -544,7 +544,11 @@ async def cmd_stop(bot, message):
     else:
         ps["current"] = None
         ps["task"] = None
+        ps["done_event"] = None
         ps["duck_vol"] = None
+        ps["duck_task"] = None
+        ps["progress"] = None
+        ps["cur_seek"] = 0.0
 
     await bot.reply(message, "Stopped")