feat: let AuthClient take identity and delegation storage

AuthClient now takes identityStorage and delegationStorage instead of one
combined store, which is what lets an application share sign-in state across
sibling subdomains by handing it CookieDelegationStorage. The client is
key-type agnostic as a result: the keyType option, the pending-key machinery,
key (de)serialization, StoredKey and the localStorage migration are all gone,
because the choice of algorithm now belongs to the identity store.

The window flow persists the key only once a delegation is obtained, so a
cancelled or failed ceremony can no longer overwrite the key of a session that
is still valid. The redirect flow has to persist on the outbound load, since the
key must survive the navigation, so #hydrate and #reconcile verify that the
stored key matches the delegation's leaf public key and discard the session on a
mismatch rather than assembling an identity that signs with an unauthorized key.

subscribe() and dispose() expose the stores' change events and re-derive the
identity on an external sign-in or sign-out. isAuthenticated() is synchronous,
reading the delegation's expiry directly. Each concrete store owns its own
storage slot, so no slot-key constants are exported.

BREAKING CHANGE: removes the keyType option and the AuthClientStorage and
StoredKey interfaces, along with the IdbStorage, LocalStorage and KEY_STORAGE_*
exports; renames the storage option to identityStorage. Existing sessions are
not migrated and users re-authenticate.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: sea-snake