fix(settings): isolate self-test configuration

[lukisch]

Summary

• isolate the complete python main.py --self-test lifecycle in temporary APPDATA
• preserve the user's real %APPDATA%\CleanMarkdown\settings.json byte-for-byte and mtime-for-mtime
• cover all 13 settings through save/restart/load and correct JSON storage documentation
• retain current upstream documentation while reconciling the test inventory to 122

Verification

• python -m pytest -q -p no:cacheprovider — 122 passed
• python main.py --self-test — 53 checks, exit 0
• real settings SHA-256 and mtime unchanged across the self-test
• python -m py_compile main.py translator.py manage_translations.py
• python -m ruff check .
• git diff --check

Ticket: T-20260731-24