fractions add/sub fix

lib/math/validation.ts

@@ -15,6 +15,20 @@ export function parseStrictInt(input: string): number | null {
   return parseInt(s, 10);
 }
 
+/** Parse a strict signed integer string. Reject decimals, scientific notation, and trailing text. */
+export function parseStrictSignedInt(input: string): number | null {
+  const s = input.trim();
+  if (!/^-?\d+$/.test(s)) return null;
+  return parseInt(s, 10);
+}
+
+/** Parse a strict signed decimal string. Reject scientific notation and trailing text. */
+export function parseStrictSignedDecimal(input: string): number | null {
+  const s = input.trim();
+  if (!/^-?\d+(\.\d+)?$/.test(s)) return null;
+  return parseFloat(s);
+}
+
 export type AnswerResult =
   | { correct: true; simplified: boolean }
   | { correct: false; message: string };
@@ -37,6 +51,11 @@ export function checkFractionAnswer(
     return { correct: false, message: "That's not quite right. Try again!" };
   }
 
+  // Any non-zero denominator gives the same value for zero, so don't force 0/1 in the UI.
+  if (userNum === 0) {
+    return { correct: true, simplified: true };
+  }
+
   const isSimplified = gcd(Math.abs(userNum), Math.abs(userDen)) === 1;
 
   if (requireSimplified && !isSimplified) {