fix: Move base_plugin.py to core module and update imports

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.

core/plugin_manager.py

@@ -12,7 +12,7 @@ import importlib.util
 from pathlib import Path
 from typing import Dict, List, Type, Optional
 
-from plugins.base_plugin import BasePlugin
+from core.base_plugin import BasePlugin
 
 
 class PluginManager:

core/plugin_manager_optimized.py

@@ -20,7 +20,7 @@ from typing import Dict, List, Type, Optional, Any
 from concurrent.futures import ThreadPoolExecutor, as_completed
 from functools import lru_cache
 
-from plugins.base_plugin import BasePlugin
+from core.base_plugin import BasePlugin
 
 
 class PluginMetadata:

plugins/plugin_store_ui/plugin.py

@@ -6,7 +6,7 @@ Provides the Plugin Store interface for browsing and installing plugins.
 
 from PyQt6.QtWidgets import QWidget, QVBoxLayout, QLabel
 
-from plugins.base_plugin import BasePlugin
+from core.base_plugin import BasePlugin
 from core.plugin_store import PluginStoreUI
 
 

plugins/settings/plugin.py

@@ -13,7 +13,7 @@ from PyQt6.QtWidgets import (
 from PyQt6.QtCore import Qt, QTimer
 
 from core.settings import get_settings
-from plugins.base_plugin import BasePlugin
+from core.base_plugin import BasePlugin
 
 
 class SettingsPlugin(BasePlugin):