Add validation in NewJWTHS256 constructor

As discussed in [PR #8](https://github.com/itsLeonB/time-tracker/pull/8), we need to add validation in the `NewJWTHS256` constructor to ensure that `secretKey` and `issuer` are not empty, and `tokenDuration` is positive.

Referenced comment: [Link](https://github.com/itsLeonB/time-tracker/pull/8#discussion_r1882439834)

Requester: @itsLeonB