Show full calculation in wrong-answer feedback

On an incorrect answer, display "❌ 15 + 9 = 24" (the question expression
plus the correct answer) instead of just "❌ Answer: 24", so kids see the
whole equation. Captures state["question"] before it's overwritten.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app.py

@@ -303,7 +303,8 @@ def check_answer(user_input: str, state: dict, pre_image, pre_data_url: str, col
                         gr.update(), gr.update(), gr.update(), gr.update(), gr.update())
 
         else:
-            correct_answer   = state["answer"]   # capture before overwrite
+            correct_answer   = state["answer"]      # capture before overwrite
+            current_q        = state["question"]    # the expression, e.g. "15 + 9"
             state["streak"]  = 0
             math_lv          = _math_level(state)
             question, answer = generate_question(math_lv)
@@ -311,7 +312,7 @@ def check_answer(user_input: str, state: dict, pre_image, pre_data_url: str, col
             state["answer"]   = answer
             return (state, _status_text(state),
                     gr.update(value=_safe_question(state)), gr.update(value=""),
-                    f"❌ Answer: **{correct_answer}**",
+                    f"❌ {current_q} = **{correct_answer}**",
                     gr.update(visible=False), "", gr.update(visible=False), "",
                     gr.update(), gr.update(), gr.update(), gr.update(), gr.update())