Authentication: Email Login magic link flow #115

Describe the current behavior
Users can only log in via Google OAuth or by manually adding an X-API key through the Keystore page. There is no way to log in with just an email address, which limits access for users who don't have Google accounts or prefer email-based authentication.

Describe the enhancement you'd like
Add a magic link login flow that allows users to sign in by entering their email address:

1. Login Modal: Add an email input field with a "Continue" button below the Google login (separated by an "OR" divider). On submit, call POST /api/v1/auth/magic-link with { "email": "[email protected]" }. On success, show a "Check your email" confirmation. On error (e.g. "No account found for this email"), show the error inline under the email field.

2. Verify Page (/verify?token=xxx): When the user clicks the login link in their email, they land on this page. It calls GET /api/v1/auth/magic-link/verify?token=xxx to validate the token, set cookies, and log the user in. On success, redirect to the dashboard. On failure, show the error with a retry option.