The base_plugin.py was removed from plugins/ folder during cleanup
but is still needed by core/plugin_manager.py and built-in plugins.
CHANGES:
- Restored base_plugin.py from plugin repo to core/base_plugin.py
- Updated imports in core/plugin_manager.py
- Updated imports in core/plugin_manager_optimized.py
- Updated imports in plugins/settings/plugin.py
- Updated imports in plugins/plugin_store_ui/plugin.py
This fixes ModuleNotFoundError when starting EU-Utility.