Add plugin system with sample plugin and settings UI

Introduces a plugin architecture, including a PluginManager, plugin loading in main and renderer processes, and a sample plugin demonstrating menu, IPC, and context menu contributions. Adds a Plugins tab to the settings UI for managing plugins (enable/disable, reload), and updates preload.js to load renderer preloads from plugins. Documentation for plugin development is included in README-PLUGINS.md.

plugins/sample-hello/plugin.json

@@ -0,0 +1,9 @@
+{
+  "id": "sample-hello",
+  "name": "Sample Hello Plugin",
+  "version": "0.1.0",
+  "description": "Demonstrates Nebula plugin basics: add menu item and renderer API.",
+  "main": "main.js",
+  "rendererPreload": "renderer-preload.js",
+  "enabled": false
+}