fix(engine): toast on challenge exhaustion, block input during auto-complete (#71, #72)

- challenge_plugin: replace silent warn+return with InfoToast when all
  challenges are completed so the player gets clear feedback (#72)
- input_plugin: add AutoCompleteState guard to start_drag,
  touch_start_drag, and handle_double_tap so player input cannot race
  with the auto-complete move sequence (#71)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

solitaire_engine/src/challenge_plugin.rs

@@ -93,7 +93,9 @@ fn handle_start_challenge_request(
         return;
     }
     let Some(seed) = challenge_seed_for(progress.0.challenge_index) else {
-        warn!("challenge seed list is empty");
+        info_toast.write(InfoToastEvent(
+            "You've completed all challenges! More coming soon.".into(),
+        ));
         return;
     };
     new_game.write(NewGameRequestEvent {