add ruff config and clean unused imports

[SaInekK]

Description

Adds a pragmatic ruff configuration to pyproject.toml and removes unused imports flagged by it across 7 files. No runtime behaviour changes.

The ruff config is intentionally conservative for a legacy research codebase:

• enables pyflakes (F) and syntax-errors (E9) — real bugs only
• ignores stylistic noise that would require touching many files for no functional gain (F403, F405, F841)
• per-file ignores for __init__.py re-exports and tests/*

Adds ruff to the dev optional-deps so contributors get it via pip install -e ".[dev]".

Fixes # (no specific issue — infrastructure)

Type of change

• New feature (non-breaking change which adds functionality) — tooling only

Testing

• pytest tests/ --ignore=tests/test_visualize_questions_gui.py → passes (6 unrelated pre-existing CSV failures on main are addressed by sibling PR for pd.read_csv(sep=None) deprecation)
• ruff check src tests → all checks passed

Test Configuration

• Library version: this branch
• OS: macOS 14
• Toolchain: Python 3.11, ruff 0.x

Checklist

• My PR is for one issue, rather than for multiple unrelated fixes.
• My code follows the style guidelines of this project.
• I have performed a self-review of my own code
• My changes generate no new warnings
• New and existing unit tests pass locally with my changes