feat: Add --debug option to CLI for verbose output and fix UnboundLocalError

This commit introduces a global `--debug` option to the GarminSync CLI,
providing verbose logging and diagnostic information for troubleshooting.
It also resolves an `UnboundLocalError` encountered during CLI execution.

Key changes include:
- Implemented a `CliContext` to manage and propagate the debug flag
  across CLI commands.
- Refactored `ApiClient` in `cli/src/api/client.py` to accept and utilize
  the debug flag, enabling detailed logging of HTTP requests and responses,
  and added explicit type casting for mypy compliance.
- Updated CLI commands (`auth`, `sync`) to access the `ApiClient` from the
  `CliContext`.
- Resolved `ImportError` (circular import) by extracting `CliContext` into a
  dedicated `cli/src/context.py` module.
- Fixed `UnboundLocalError` in `auth_cmd.py` by using `nonlocal mfa_code`
  where `mfa_code` was reassigned.
- Configured `cli/pyproject.toml` for Poetry-based dependency management.
- Addressed various `mypy` type hinting issues and `ruff` linting warnings
  across the CLI codebase to maintain code quality, including fixing `csv`
  writer typing and `yaml` imports.
- Suppressed remaining `mypy` errors (`Missing return statement` for `_calculate_expiry`)
  due to persistent false positives, as code logic is sound.