fix(webui): allow large chapter forms

[hydraxman]

Summary

• raise Flask's in-memory form and multipart field limits for books with many chapters
• cover both URL-encoded and multipart wizard submissions with regression tests
• require Flask 3.1+, where these form-limit configuration keys are supported

Problem

The chapter wizard submits several fields per chapter. Large books can exceed Werkzeug's default form memory or multipart field limits and receive HTTP 413 before /wizard/update runs, even though the request is far below the existing 400 MB upload limit.

Testing

• .venv/bin/python -m pytest -q tests/test_webui_form_limits.py — 2 passed
• uv tool run ruff check tests/test_webui_form_limits.py — passed
• python3 -m compileall -q abogen/webui/app.py tests/test_webui_form_limits.py — passed
• git diff --check — passed

Fixes #167