fix(fxa-settings): skip passwordless OTP redirect for cached sessions

[vbudhram]

Because

• Passwordless users with a valid cached session (sessionToken in localStorage) were redirected to the OTP code page when navigating to / or /signin
• This happened via both the Index container (email-first auto-submit) and the Signin container (direct navigation), forcing unnecessary re-authentication

This pull request

• Adds !sessionToken guard in Signin/container.tsx to skip the passwordless OTP redirect when a cached session exists
• Adds hasCachedSession check in Index/container.tsx handleSuccessNavigation — reads currentAccount() fresh from localStorage at call time to detect cached sessions
• Adds cachedAccount variable in Index container to consolidate currentAccount() || lastStoredAccount() for suggestedEmail
• Adds passwordless OTP header logging in mail_helper.js (x-passwordless-signup-otp, x-passwordless-signin-otp)
• Adds unit tests in both Index/container.test.tsx and Signin/container.test.tsx for cached session behavior
• Adds Relay browser service functional test

Fixes: #20195

Checklist

• My commit is GPG signed
• Tests pass locally (if applicable)
• Documentation updated (if applicable)
• RTL rendering verified (if UI changed)

Other Information

How to test:

1. Create a passwordless account via 123done with force_passwordless=true
2. Complete OTP flow, confirm you're logged in
3. Navigate to http://localhost:3030/ — should see cached signin page (avatar + email), NOT the OTP code page
4. Navigate to http://localhost:3030/signin?email=<your-email> — same behavior

[StaberindeZA]

lgtm! ty