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/renderer-preload.js

@@ -0,0 +1,8 @@
+// Renderer preload for sample plugin
+// You can expose new APIs to the page
+const { contextBridge, ipcRenderer } = require('electron');
+
+contextBridge.exposeInMainWorld('sampleHello', {
+  ping: () => ipcRenderer.invoke('sample-hello:ping'),
+  onHello: (handler) => ipcRenderer.on('sample-hello', (_e, payload) => handler(payload))
+});