Because:
- The Index page sent every pairing-authority integration to the v1
/pair/auth/allow screen, so a v2 desktop flow that reached `/` landed
on the legacy pair screens.
- The integration had no record of which pairing version it was built
for until a channel was created.
This commit:
- Adds isDevicePairingAsV2Authority() to the integration flags.
- Passes the pairing version into PairingAuthorityIntegration and
exposes it as pairingVersion.
- Routes a v2 authority from Index to /pair/authority/scan_qr, replacing
the history entry; v1 still goes to /pair/auth/allow.
- Adds tests for the flag, factory, model, and both Index redirects.
Fixes FXA-14464