feat: add global --no-color flag and NO_COLOR handling for CLI

[fmueller]

Motivation

• Provide a single, consistent way to disable colored CLI output for all commands and respect the common NO_COLOR environment variable.

Description

• Add a global --no-color option to the Typer app callback in src/scribae/main.py and check NO_COLOR in os.environ to control coloring at runtime.
• Use click.get_current_context(...).color = False to disable ANSI coloring when requested, and add import click and import os to support this behavior.
• Add three unit tests in tests/unit/main_test.py to validate colored output, --no-color flag behavior, and NO_COLOR environment handling, and ensure tests unset NO_COLOR during setup with monkeypatch.delenv.

Testing

• Ran linting with uv run ruff check src tests, which passed.
• Ran type checks with uv run mypy src, which failed due to unrelated missing stubs for torch in src/scribae/translate/mt.py (2 errors reported).
• Ran unit tests with uv run pytest, which passed: 163 passed.

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Codex Task