feat(frontend): wire frontend to backend pow API

[DecentAgeCoder]

feat(frontend): wire frontend to backend pow API

Motivation

• The Pull Request addresses the improvements that have already been discussed before splitting the PR. The backend API for createPowChallenge and allowSigning return the response object and not the result, makes the nonce parameter optional for allowSigning, and and refining error handling for better reliability.

Changes

• allowSigning() can now be called without any parameters
• Removed AllowSigningResult and CreateChallengeResult since they are not needed any longer
• Refactored createPowChallenge and allowSigning functions to improve clarity and reliability.
• Introduced dedicated error handling for CreateChallengeError
• Updated test implementations for createPowChallenge and allowSigning to reflect changes.
• Simplified test cases by focusing on updated logic and removing obsolete portions.

Tests

• Updated tests for allowSigning to match new behavior with simplified parameters.
• Enhanced createPowChallenge tests to verify proper error handling for specific error cases (e.g., "Challenge in Progress," missing user profile, randomness error).

[AntonioVentilii]

May you please split it between changes for createPowChallenge and changes to allowSigning?

Furthermore, i see that you are removing allowSigningResult: this should go in a separate PR