Unlink a sign-in identity from an account

Follow-up to #70, which added identity linking (attach an email/phone to the signed-in account) and account merge. Unlinking — removing a sign-in method from the profile — was left out of that issue's scope.

## Requirements

- A signed-in user can remove one of their identities (email, phone; passkeys already have their own lifecycle via the identity rows).
- **Last-identity guard**: refuse to remove the only remaining identity so a user cannot strand their account with no way to sign in. Decide whether passkey identities count toward "remaining" (a passkey-only account is usable, but losing the device strands it).
- Probably requires a fresh session or re-verification for removing high-value identifiers, mirroring the caution in the linking flow.
- `IdentityStore.delete` already exists (optional) as the storage primitive.
- Adapter/UI: a remove action on the sign-in methods list the example app's dashboard now renders.