fix(ui): check ApiError.status instead of message string for 404 detection

The ApiError class (088bc34) extracts messages from response bodies, so
a 404 with no body produces "Request failed" — not "404". The email
connector hook's string check failed, treating "not configured" as an
error and showing "Failed to load config" on fresh installs.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/api/email-connector-hooks.ts

@@ -1,5 +1,5 @@
 import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query';
-import { api } from './client';
+import { api, ApiError } from './client';
 
 export interface EmailConnectorResponse {
   connectorId: string;
@@ -32,7 +32,7 @@ export function useEmailConnector() {
       try {
         return await api.get<EmailConnectorResponse>('/vendor/email-connector');
       } catch (e) {
-        if (e instanceof Error && e.message.includes('404')) return null;
+        if (e instanceof ApiError && e.status === 404) return null;
         throw e;
       }
     },