Both restore_suffixed_filenames.py and migrate_legacy_metadata.py
hardcoded Path.home() / '.config' / APP_NAME for finding settings.json,
which only works on Linux. On Windows this resolves to the wrong path
(~/.config/ instead of %LOCALAPPDATA%).
Replace the hand-rolled fallback with platformdirs.user_config_dir(),
which correctly resolves to the OS-appropriate config directory on all
platforms (Windows: %%LOCALAPPDATA%%, macOS: ~/Library/Application Support,
Linux: ~/.config). The portable mode check (settings.json in repo root
with use_portable_settings: true) is preserved unchanged.