fix: harden twenty four game submissions

backend/static/js/games.js

@@ -143,6 +143,13 @@
     input.placeholder = "例如：6 / (1 - 3 / 4)";
     input.autocomplete = "off";
     input.inputMode = "text";
+    input.spellcheck = false;
+    const errorMessage = document.createElement("p");
+    errorMessage.className = "form-error game-form-error";
+    errorMessage.setAttribute("role", "alert");
+    input.addEventListener("input", () => {
+      errorMessage.textContent = "";
+    });
     const keypad = document.createElement("div");
     keypad.className = "game-keypad";
     ["+", "-", "*", "/", "(", ")"].forEach((operator) => {
@@ -159,10 +166,11 @@
     submit.className = "primary-button";
     submit.type = "submit";
     submit.textContent = "验证并计分";
-    form.append(input, keypad, submit);
+    form.append(input, keypad, errorMessage, submit);
     form.addEventListener("submit", async (event) => {
       event.preventDefault();
       submit.disabled = true;
+      errorMessage.textContent = "";
       try {
         const result = await api(
           `contests/games/attempts/${attempt.attempt_id}/submit/`,
@@ -175,6 +183,7 @@
         renderTwentyFour(result);
         showToast("得到 24，成绩已记录");
       } catch (error) {
+        errorMessage.textContent = error.message;
         showToast(error.message);
         submit.disabled = false;
       }