feat: challenge completed

[Theo6890]

There are some things that must be done, if such a contract would be used in production:

• optimize pendingRewards and extensive testing as this is the critical logic part
• fix totalRewards counter issues
• more refactoring with more helpers
• deeper testing: fuzz testing, specific edges cases, static analysers & write a document for test scenarios + code review on paper for critical points (pendingRewards)
• break down the app in tasks and branches instead of working on a single branch in order to have the choice to revert a whole feature once merge in main

Notes: some config files have put outside project as it is not possible to do differently (.github, .husky...)