Refactor/replace zerolog with slog

[amrshaban2005]

Summary

Replaces zerolog usage with Go's standard log/slog.

This migration centralizes logger setup in the logger package, configures slog as the default logger, and updates call sites to use package-level slog logging instead of passing logger instances through structs and functions.

Changes

• Add slog-based logger setup.
• Add a centralized fatal logging helper.
• Replace zerolog chained calls with structured slog calls.
• Remove logger fields from structs where logging can use the global slog default.
• Update tests and setup code for the new logging approach.

Validation

• make test-local
• make test-ci-local-clean

Related issue

Closes #122