feat(integrations): add Discogs artwork fetch and fix Last.fm env loading

- Add Discogs API client for fetching missing album artwork
- Add UI button in Local Library settings to fetch missing artwork
- Load .env file at runtime using dotenvy for development
- Fix Last.fm to check runtime env vars as fallback to compile-time
- Use XDG Portal for dialog plugin (cross-desktop compatibility)
- Update README and .env.example with Discogs configuration

.env.example

@@ -6,3 +6,10 @@ LAST_FM_API_KEY=your_api_key_here
 LAST_FM_API_SHARED_SECRET=your_shared_secret_here
 LAST_FM_APP_NAME=your_app_name
 LAST_FM_APP_REGISTER=your_lastfm_username
+
+# Discogs API Configuration
+# Create your own Discogs application at: https://www.discogs.com/settings/developers
+# These credentials are used for fetching album artwork
+
+DISCOGS_API_CLIENT_KEY=your_consumer_key_here
+DISCOGS_API_CLIENT_SECRET=your_consumer_secret_here