feat(#417): update auth-client.ts error messages to PDA-friendly

Uses parseAuthError from auth-errors module for consistent
PDA-friendly error messages in signInWithCredentials.

Refs #417

apps/web/src/lib/auth-client.ts

@@ -9,6 +9,7 @@
 import { createAuthClient } from "better-auth/react";
 import { genericOAuthClient } from "better-auth/client/plugins";
 import { API_BASE_URL } from "./config";
+import { parseAuthError } from "./auth/auth-errors";
 
 /**
  * Auth client instance configured for Mosaic Stack.
@@ -42,8 +43,9 @@ export async function signInWithCredentials(username: string, password: string):
   });
 
   if (!response.ok) {
-    const error = (await response.json().catch(() => ({}))) as { message?: string };
-    throw new Error(error.message ?? "Authentication failed");
+    const errorBody = (await response.json().catch(() => ({}))) as { message?: string };
+    const parsed = parseAuthError(errorBody.message ? new Error(errorBody.message) : response);
+    throw new Error(parsed.message);
   }
 
   const data = (await response.json()) as unknown;