Because:
We need to wire the passkey WebAuthn ceremony to the passkey button
introduced in FXA-13487.
This commit:
* Adds a usePasskeySignIn hook driving the ceremony, error handling,
Sync merge gate, and post-auth routing, and wires it into the Index
and Signin pages.
* Mounts /signin/passkey/fallback as a temporary stand-in until
FXA-13100 lands the real password-fallback page.
* Fixes the auth-server passkey route to return sessionToken.data
instead of .id so the client can Hawk-sign subsequent requests.
* Enables passkeyAuthenticationEnabled in local.json-dist.
* Adds functional tests covering the passkey sign-in happy, cancel,
and PASSKEY_NOT_FOUND paths.
Closes #FXA-13099