use ServerQueryInput properly

packages/frontend/src/lib/server/user/byUsername.ts

@@ -8,7 +8,7 @@ export async function byUsername({
     onError,
 }: {
     username: string;
-} & ServerQueryInput) {
+} & ServerQueryInput<UserRecord>) {
     const url = new URL(`${getServerURL()}/user/by-username`);
     url.searchParams.set("username", username);
 
@@ -17,8 +17,10 @@ export async function byUsername({
     });
 
     if (!res.ok) {
-        const error = await res.text();
-        onError?.(error || `failed to get user (${res.status})`);
+        const error = await res.json().catch(() => res.text());
+        const message =
+            typeof error === "string" ? error : error.error || `failed to get user (${res.status})`;
+        onError?.(message);
     } else {
         const data = (await res.json()) as UserRecord;
         onSuccess?.(data, res);