fix(settings): send can_link_account with stored uid in email-first
Because: * Newer Firefox got can_link_account only after sign-in, so a retry showed the merge warning again. This commit: * Sends can_link_account with the stored uid from email-first. * Passes the approved uid to signin and signin_unblock. * Skips the post-auth prompt there only when the real uid matches. Closes FXA-13012