feat: add login page and auth callback route

- Replace default Next.js home with role-aware root redirect
- Add email/password login form (client component, signInWithPassword)
- Add auth callback route for Supabase code exchange
- Login page at /login, callback at /api/auth/callback

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AWxyMibCuGGtSQSqfajDQ7

app/(auth)/login/page.tsx

@@ -0,0 +1,12 @@
+// app/(auth)/login/page.tsx
+import { LoginForm } from '@/components/auth/login-form'
+
+export default function LoginPage() {
+  return (
+    <main className="min-h-screen flex items-center justify-center bg-gray-50 p-4">
+      <div className="bg-white rounded-xl shadow-sm border p-8 w-full max-w-sm">
+        <LoginForm />
+      </div>
+    </main>
+  )
+}