Avoid accidental graph edits on click

[bhack]

Summary

• keep graph clicks selection-only until drag movement passes the GTK drag threshold
• anchor graph drags to the original band point so points do not jump to the click location
• add graph interaction tests for zero-offset drag, below-threshold movement, and anchored dragging

Validation

• .venv/bin/python -m pytest tests/test_mini_eq_window_graph.py -q
• .venv/bin/python -m ruff check .
• .venv/bin/python -m ruff format --check .
• .venv/bin/python -m pytest -q