Cache library artwork

[ghost]

• Adds StoreArtworkCache for per-store/per-game artwork files keyed by slot and source URL hash.
• Preloads Steam, Epic, and GOG library artwork through the shared cache and reuses cached files across app restarts.
• Deletes cached artwork folders for Steam, Epic, and GOG games when the game is uninstalled or removed.
• Suppresses remote cache downloads for slots replaced by custom library artwork and deletes stale cached remote files for those slots.
• Restores remote cache population automatically after custom artwork is cleared.
• Parses GOG background/screenshot hero artwork, persists hero_image_url, and migrates existing Room databases to schema version 7.
• Routes library cards and detail/store hero images through cached artwork models when available.