Fix false custom warning for default Fix heading detection

[Copilot]

Summary

Fixes a regression where a default ACB Fix run could be labeled as using custom settings.

What changed

• Updated web/src/acb_large_print_web/customization_warning.py so default heading detection behavior is treated as baseline:
  • detect_headings=True with heading_accuracy=balanced is no longer flagged as custom.
  • detect_headings=False is now explicitly flagged as a customization.
  • Non-default heading accuracy modes remain flagged as custom.
• Updated web/tests/test_customization_warning.py:
  • Default-fix-options test now reflects default heading detection behavior.
  • Added a regression test ensuring disabled heading detection is reported as a customization.
• Updated CHANGELOG.md under [Unreleased] with the fix details.

Validation

• python -m ruff check web/src/acb_large_print_web/customization_warning.py web/tests/test_customization_warning.py
• PYTHONPATH=/home/runner/work/glow/glow/web/src:/home/runner/work/glow/glow/desktop/src python -m pytest web/tests/test_customization_warning.py -q --noconftest
• parallel_validation (Code Review + CodeQL): passed with no findings

Notes

• Full web test environment could not be installed via web package in this sandbox because the runtime here is Python 3.12 while web/pyproject.toml requires Python >=3.13.