feat(frappe-api): implement StorageAuthService for persistent credentials storage (#38)

[rohansaini-02]

Summary

This PR implements the StorageAuthService in poc-frappe-api to persist authentication credentials using an injected storage provider interface, ensuring users stay logged in across application restarts.

Related Issue

Resolves #38

Changes Made

• Storage Abstraction: Defined the IStorageProvider interface with support for both synchronous and asynchronous operations (getItem, setItem, removeItem).
• Persistent Auth Service: Added StorageAuthService to serialize credentials to and from the storage provider under a custom key.
• Robustness: Ensured storage read operations fail gracefully (returning null) when the storage is empty, fields are missing, or serialization payload is corrupted.
• Test Coverage: Added a complete suite of unit tests in AuthService.test.ts testing the persistent storage implementation against both synchronous and asynchronous mock providers.

Testing Performed

• Installed dependencies and ran the test runner in poc-frappe-api:

  npm test

  All 9 tests in AuthService.test.ts and FrappeClient.test.ts passed successfully.