Added mock tests and refactured the entire code

[RadomirTikkerbar]

No description provided.

[github-actions]

API & Mocked Tests and Coverage

Purpose: Validate README requirements for regular tests, mocked email tests, coverage, and runtime comparison.
Status: ✅ PASS
Regular tests (app.test.js): ✅ PASS (7319ms)
Mocked tests (app.mock.test.js): ✅ PASS (1291ms)
Coverage gate (both runs at 100%): ✅ PASS
Runtime gate (regular ≥ mocked + 1500ms): ✅ PASS
Regular coverage (app.js): lines=100% | statements=100% | functions=100% | branches=100%
Mocked coverage (app.js only): lines=100% | statements=100% | functions=100% | branches=100%
Run details: https://github.com/tanjaq/User-Register/actions/runs/23716555130
PR checks: https://github.com/tanjaq/User-Register/pull/29/checks

[tanjaq]

Task 1 - Not done, no additional tests added.
Task 2 - OK, but validations are shallow.

@RadomirTikkerbar