LG-15672: Only redirect to select email screen if feature enabled

[aduth]

🎫 Ticket

LG-15672

🛠 Summary of changes

Fixes an issue where users may be redirected to the email selection experience even if the feature is disabled, resulting in a 404.

Related Slack thread: https://gsa-tts.slack.com/archives/C01710KMYUB/p1738779905418149

📜 Testing Plan

Test with and without feature_select_email_to_share_enabled enabled:

1. Have sample application running in separate process
2. Go to http://localhost:9292
3. Click "Sign in"
4. Sign in or create an account
5. After returned to sample application, go to http://localhost:3000
6. Add email address from account dashboard
7. Confirm email address
8. Observe that:
   • If feature is disabled, you're redirected to account dashboard with email selected
   • If feature is enabled, you're redirected to email selection screen

Previously, this would always redirect to email selection screen regardless if feature is enabled.

Follow-up work should explore whether email selection screen is the correct place to redirect in this workflow.

[jmdembe]

Everything here looks good to me!