Fix Auth0 500 error when not configured (Issue #5)

- Add check for AUTH0 credentials before attempting login
- Show friendly error message if Auth0 not configured
- Hide Auth0 button on login page when not configured
- Add try/catch for auth0.authorize_redirect() failures

Fixes #5

templates/login.html

@@ -48,6 +48,7 @@
             <span>or</span>
         </div>
 
+        {% if auth0_configured %}
         <div class="social-auth-buttons">
             <a href="{{ url_for('auth0_login') }}" class="social-btn auth0-btn">
                 <svg width="20" height="20" viewBox="0 0 24 24" fill="currentColor">
@@ -56,6 +57,7 @@
                 Continue with Auth0
             </a>
         </div>
+        {% endif %}
 
         <div class="auth-footer">
             <p>Don't have an account? <a href="{{ url_for('signup') }}">Sign up</a></p>