refactor: mfa un-pairing redesign

Overview

B.1. On unpair success: (1) navigate user to success page (like MFA pairing success) with appropriately different text.

B.2. The unpair UI should be: (1) in sections like MFA pairing, (2) two steps laid out like MFA pairing, (3) help message at bottom middle like in MFA pairing. (4) "If you lost your phone…" should say "Alternatively, unpair via email."

Extra: Loading state for unpairing "Send Token" button.

TUP-549
builds upon #288
causes #289

Changes

added two new routes mfa/unpair and mfa/unpair/success
changed MfaWrapper to distinguish paring vs un-pairing
moved MFA unpairing out of ManageAccoutnMfa into new MfaUnpairingView
added heading above loading spinner for Manage Account MFA block
moved steps-as-panels CSS into new component c-step-panels[^1]
added style to center align MFA message text

Testing

Pair and Unpair with QR and CMS.

UI

Shared via internal communication for data privacy.

Design approved via internal communication.

[^1]: The CSS was originally extracted into a component cuz #288 needed the same styles in modal and not-modal. I retained here, to keep panel layout styles isolated from styles for content within panel. If this just adds more confusion, please tell me, and I will revert.

TACC / tup-ui