Add "Put Back" button to cancel accidental discard draws

When you accidentally click the discard pile, you can now put the card
back instead of being forced to swap. The "Put Back" button appears
only when you've drawn from the discard pile.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

server/main.py

@@ -748,6 +748,14 @@ async def websocket_endpoint(websocket: WebSocket):
                             # Turn ended, check for CPU
                             await check_and_run_cpu_turn(current_room)
 
+            elif msg_type == "cancel_draw":
+                if not current_room:
+                    continue
+
+                async with current_room.game_lock:
+                    if current_room.game.cancel_discard_draw(player_id):
+                        await broadcast_game_state(current_room)
+
             elif msg_type == "flip_card":
                 if not current_room:
                     continue