Adds a minimal and lean configurable logging using spdlog (introduced as a new dep), replacing direct console output but preserving the old log file formatting.

Checklist:

• I made sure that the CI passed before I ask for a review.
• I added a summary of the changes (compared to the last release) in the CHANGELOG.md.
• If necessary, I made changes to the documentation and/or added new content.
• I will remember to squash-and-merge, providing a useful summary of the changes of this PR.