fix: route browser→API calls through BFF to avoid CORS

- Create /api/onboard/request-otp and /api/auth/member/login BFF routes
- Update OnboardingForm and member-login page to call BFF instead of API directly
- Add vercel.json with monorepo build config and rootDirectory hint
- Add .vercelignore to exclude .turbo, node_modules, .next (was uploading 2.3GB)
- Add apps/web/.gitignore and update root .gitignore for tsbuildinfo files

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/web/app/api/onboard/request-otp/route.ts

@@ -0,0 +1,14 @@
+import { getApiBaseUrl, jsonResponse } from '../../../_lib/api';
+
+export const dynamic = 'force-dynamic';
+
+export async function POST(req: Request): Promise<Response> {
+  const body = await req.json().catch(() => ({}));
+  const upstream = await fetch(`${getApiBaseUrl()}/public/auth/request-otp`, {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/json', Accept: 'application/json' },
+    body: JSON.stringify(body),
+    cache: 'no-store',
+  });
+  return jsonResponse(await upstream.json().catch(() => ({})), upstream.status);
+}