fix(settings): show canceled screen when desktop cancels pairing

[dschom]

Because

• Cancelling on the desktop pairing screen left the mobile device on "Looks
  like we timed out", blaming a wait the user never made.
• The designs call for the "Canceled" screen on mobile when the desktop user
  stops the flow.

This pull request

• Sends pair:auth:cancel from the authority before it tears down the pairing
  channel.
• Fails the supplicant when that notice arrives, and records that the authority
  was the one that cancelled.
• Passes reason on both supplicant exits to timeout_and_cancel, so the
  dead-end screen renders the Canceled variant.
• Adds coverage for the notice, for a send that fails, and for a bare channel
  close still reading as a timeout.

Issue that this pull request solves

Closes: FXA-14453

Checklist

Put an x in the boxes that apply

• My commit is GPG signed.
• If applicable, I have modified or added tests which pass locally.
• I have added necessary documentation (if appropriate).
• I have verified that my changes render correctly in RTL (if appropriate).
• I have manually reviewed all AI generated code.

How to review (Optional)

• Key files/areas to focus on: cancel() in pairing-authority-integration.ts
  and handleAuthCancel in pairing-supplicant-integration.ts.
• Suggested review order: the two integrations, then the three Pair2 containers.
• Risky or complex parts: the notice has to reach the supplicant before the
  channel closes; cancel() swallows a failed send so a dead channel still
  tears down.

Screenshots (Optional)

Please attach the screenshots of the changes made in case of change in user interface.

Other information (Optional)

• Supplicant-side sibling: fix(settings): show canceled screen when mobile cancels pairing #21132 (FXA-14452) fixed this same screen for the
  mobile Cancel button, and landed the "Canceled" strings this PR now reaches.
• pairingFlowV2.spec.ts covers supplicant-cancel end to end; the
  authority-cancel mirror is a follow-up. That spec is gated behind Firefox
  Nightly and does not run in CI.

[dschom]

I've manually validated this now works locally.