fix: Move viewport config to separate export per Next.js 15 requirement

- Moved themeColor and viewport config from metadata to viewport export
- Fixes Next.js 15 warnings about unsupported metadata fields
- Registration error was actually due to user already existing (409)
- Backend validation passes correctly with deviceInfo included

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

maternal-web/app/layout.tsx

@@ -1,4 +1,4 @@
-import type { Metadata } from 'next';
+import type { Metadata, Viewport } from 'next';
 import { Inter } from 'next/font/google';
 import { ThemeRegistry } from '@/components/ThemeRegistry';
 import { ErrorBoundary } from '@/components/common/ErrorBoundary';
@@ -16,17 +16,18 @@ import './globals.css';
 
 const inter = Inter({ subsets: ['latin'] });
 
+export const viewport: Viewport = {
+  width: 'device-width',
+  initialScale: 1,
+  maximumScale: 1,
+  userScalable: false,
+  themeColor: '#FFB6C1',
+};
+
 export const metadata: Metadata = {
   title: 'Maternal - AI-Powered Child Care Assistant',
   description: 'Track, analyze, and get AI-powered insights for your child\'s development, sleep, feeding, and more.',
   manifest: '/manifest.json',
-  themeColor: '#FFB6C1',
-  viewport: {
-    width: 'device-width',
-    initialScale: 1,
-    maximumScale: 1,
-    userScalable: false,
-  },
   appleWebApp: {
     capable: true,
     statusBarStyle: 'default',