Add biometric login button to login page

- Add biometric authentication button with Face ID/Touch ID/Windows Hello support
- Check WebAuthn support and platform authenticator availability on mount
- Handle biometric login flow with proper error handling
- Show biometric button only when device supports it
- Add loading states and user-friendly error messages

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

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

maternal-web/app/(auth)/login/page.tsx

@@ -1,6 +1,6 @@
 'use client';
 
-import { useState } from 'react';
+import { useState, useEffect } from 'react';
 import { useRouter } from 'next/navigation';
 import {
   Box,
@@ -15,7 +15,7 @@ import {
   CircularProgress,
   Link as MuiLink,
 } from '@mui/material';
-import { Visibility, VisibilityOff, Google, Apple } from '@mui/icons-material';
+import { Visibility, VisibilityOff, Google, Apple, Fingerprint } from '@mui/icons-material';
 import { useForm } from 'react-hook-form';
 import { zodResolver } from '@hookform/resolvers/zod';
 import { motion } from 'framer-motion';
@@ -23,6 +23,8 @@ import * as z from 'zod';
 import { useAuth } from '@/lib/auth/AuthContext';
 import { MFAVerificationDialog } from '@/components/auth/MFAVerificationDialog';
 import { tokenStorage } from '@/lib/utils/tokenStorage';
+import { biometricApi } from '@/lib/api/biometric';
+import { startAuthentication } from '@simplewebauthn/browser';
 import Link from 'next/link';
 
 const loginSchema = z.object({
@@ -36,6 +38,8 @@ export default function LoginPage() {
   const [showPassword, setShowPassword] = useState(false);
   const [error, setError] = useState<string | null>(null);
   const [isLoading, setIsLoading] = useState(false);
+  const [isBiometricLoading, setIsBiometricLoading] = useState(false);
+  const [isBiometricSupported, setIsBiometricSupported] = useState(false);
   const [mfaRequired, setMfaRequired] = useState(false);
   const [mfaData, setMfaData] = useState<{ userId: string; mfaMethod: 'totp' | 'email' } | null>(null);
   const { login } = useAuth();
@@ -45,10 +49,64 @@ export default function LoginPage() {
     register,
     handleSubmit,
     formState: { errors },
+    watch,
   } = useForm<LoginFormData>({
     resolver: zodResolver(loginSchema),
   });
 
+  const email = watch('email');
+
+  // Check biometric support on mount
+  useEffect(() => {
+    checkBiometricSupport();
+  }, []);
+
+  const checkBiometricSupport = async () => {
+    const supported = biometricApi.isSupported();
+    if (supported) {
+      const available = await biometricApi.isPlatformAuthenticatorAvailable();
+      setIsBiometricSupported(available);
+    }
+  };
+
+  const handleBiometricLogin = async () => {
+    setError(null);
+    setIsBiometricLoading(true);
+
+    try {
+      // Get authentication options from server
+      const options = await biometricApi.getAuthenticationOptions(email || undefined);
+
+      // Start WebAuthn authentication (triggers Face ID/Touch ID/Windows Hello)
+      const authenticationResponse = await startAuthentication(options);
+
+      // Send response to server for verification and get tokens
+      const result = await biometricApi.verifyAuthentication(
+        authenticationResponse,
+        email || undefined,
+        {
+          deviceId: authenticationResponse.id.substring(0, 10),
+          platform: navigator.userAgent,
+        }
+      );
+
+      // Store tokens and navigate
+      tokenStorage.setTokens(result.tokens.accessToken, result.tokens.refreshToken);
+      router.push('/');
+    } catch (err: any) {
+      console.error('Biometric login failed:', err);
+      if (err.name === 'NotAllowedError') {
+        setError('Biometric authentication was cancelled');
+      } else if (err.name === 'NotSupportedError') {
+        setError('Biometric authentication is not supported on this device');
+      } else {
+        setError(err.response?.data?.message || err.message || 'Biometric login failed. Please try again.');
+      }
+    } finally {
+      setIsBiometricLoading(false);
+    }
+  };
+
   const onSubmit = async (data: LoginFormData) => {
     setError(null);
     setIsLoading(true);
@@ -229,6 +287,20 @@ export default function LoginPage() {
             Continue with Apple
           </Button>
 
+          {isBiometricSupported && (
+            <Button
+              fullWidth
+              variant="outlined"
+              startIcon={isBiometricLoading ? <CircularProgress size={20} /> : <Fingerprint />}
+              size="large"
+              disabled={isLoading || isBiometricLoading}
+              onClick={handleBiometricLogin}
+              sx={{ mt: 2 }}
+            >
+              {isBiometricLoading ? 'Authenticating...' : 'Sign in with Biometrics'}
+            </Button>
+          )}
+
           <Box sx={{ mt: 3, textAlign: 'center' }}>
             <Typography variant="body2" color="text.secondary">
               Don't have an account?{' '}