Standardise frontend hook fetch error handling and mark Task 12 done

frontend/src/utils/fetchError.ts

@@ -0,0 +1,14 @@
+/**
+ * Normalize fetch error handling across hooks.
+ */
+export function handleFetchError(
+  err: unknown,
+  setError: (value: string | null) => void,
+  fallback: string = "Unknown error",
+): void {
+  if (err instanceof DOMException && err.name === "AbortError") {
+    return;
+  }
+
+  setError(err instanceof Error ? err.message : fallback);
+}