fix(settings): ask 2FA accounts for TOTP before v1 pairing starts
Because: - A 2FA account on v1 pairing could start pairing from /pair with no recent TOTP code; the code prompt came only after Firefox opened the channel. This commit: - Gates /pair "Continue" and "Continue to sync" on pairingRequiresTotp - Sends the user to /pair/auth/totp, which returns to /pair after a good code - Resumes pair_preferences once on return, and drops the router state - Adds unit tests and a functional test for the 2FA /pair flow Closes FXA-14781