feat: added authenticated user storage

[dovydas55]

Note

Medium Risk
Adds a new authenticated (non-E2EE) storage SDK surface that performs bearer-token HTTP requests to user-storage endpoints; correctness depends on request/response handling and error mapping for these new APIs.

Overview
Adds a new AuthenticatedUserStorage SDK client for authenticated (non-encrypted) User Storage endpoints, exposing delegations (list/create/revoke) and preferences (get/put notification settings) methods that send Bearer-authenticated fetch requests and wrap failures in UserStorageError (with 404 mapped to null for notification prefs).

Introduces the associated TypeScript API schema types, Jest+nock fixtures/mocks and tests, exports the new SDK from src/sdk/index.ts, and adds an api.spec.yaml OpenAPI spec documenting encrypted, authenticated, and internal User Storage endpoints.

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