feat: implement NPM Proxy Hosts section with reusable entity patterns

Add full Proxy Hosts feature for Control Room:
- Data layer: datasource, model with custom JSON converters for NPM API quirks
- Domain layer: ProxyHost entity with SSL, caching, websocket support
- Presentation: list page, detail/edit page, form widget

Add reusable entity page patterns for Control Room sections:
- EntityPageScaffold, EntitySection, EntitySettingRow components
- EntityForm with create/view/edit mode support
- EntityPageModeMixin for consistent mode management
- CreateOnlyField for immutable-after-creation fields

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.gitignore

@@ -12,7 +12,6 @@ pubspec.lock
 *.freezed.dart
 *.gr.dart
 *.mocks.dart
-lib/generated_plugin_registrant.dart
 
 # Keep version.g.dart - it's generated but should be committed
 # so CI/CD builds have version info without running the generator