Refactor metadata import and UI workflows; add system-scoped DAT handling, artwork ingestion, and artwork previews

- Scope No-Intro/TOSEC imports to selected local system and track import metadata
- Improve hash matching using file-level hashes with system-aware joins
- Add artwork ingest workflow with type selection, dry-run mode, and busy indicators
- Add bottom-row artwork previews (box/title/ingame) driven by selected game
- Improve UI responsiveness and layout (system-switch busy dialog, taller game list, preview row positioning)