Add Magic Link Authentication

Add magic link authentication to this app using Supabase Auth. Create a clean login page with a single email input field and a "Send me a login link" button. When submitted, call Supabase's signInWithOtp method to send the magic link email. Show a loading spinner on the button while the request is in progress, then replace the form with a confirmation message: "Check your inbox — we sent a link to [email]." If the user clicks an expired or invalid link, catch the error and display a friendly message with a "Request a new link" button that takes them back to the login form. After a successful magic link click, redirect the user to /dashboard.