feat: Add comprehensive plugin API documentation and example plugin

- Introduced PLUGIN_API.md for detailed plugin development guidelines.
- Implemented PLUGIN_SYSTEM_IMPLEMENTATION.md summarizing all requested features.
- Created example-plugin showcasing all API features including commands, message interceptors, settings, and notifications.
- Added plugin-metadata.json for the example plugin with relevant metadata.

package.json

@@ -23,6 +23,7 @@
   },
   "license": "AGPL-3.0-only",
   "dependencies": {
+    "adm-zip": "^0.5.16",
     "body-parser": "2.2.2",
     "cors": "2.8.6",
     "electron-squirrel-startup": "^1.0.1",