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/main.js

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+// Sample main-process side of a plugin
+module.exports.activate = function(ctx) {
+  ctx.log('activating');
+
+  // Add a simple menu item under Help
+  try {
+    const template = ctx.Menu.getApplicationMenu()?.items?.map(mi => mi);
+    if (template) {
+      const help = template.find(i => /help/i.test(i.label || ''));
+      const insertInto = help || template[template.length - 1];
+      if (insertInto && insertInto.submenu) {
+        insertInto.submenu.append(new ctx.Menu.MenuItem({
+          label: 'Say Hello (Sample Plugin)',
+          click: () => {
+            const win = ctx.BrowserWindow.getFocusedWindow();
+            if (win) win.webContents.send('sample-hello', { msg: 'Hello from plugin!' });
+          }
+        }));
+        ctx.Menu.setApplicationMenu(ctx.Menu.getApplicationMenu());
+      }
+    }
+  } catch (e) { ctx.warn('menu injection skipped', e); }
+
+  // Simple IPC example
+  ctx.registerIPC('sample-hello:ping', async () => ({ pong: true }));
+
+  // Optional: intercept a request (no-op demo)
+  ctx.registerWebRequest({ urls: ['*://*/*'] }, (details) => {
+    // Could cancel or redirect here, but we let it pass through
+    return { cancel: false };
+  });
+
+  // Context menu contribution example
+  ctx.contributeContextMenu?.((template, params, sender) => {
+    template.push({ type: 'separator' });
+    template.push({
+      label: 'Sample: Greet Console',
+      click: () => {
+        try { (sender.hostWebContents || sender).executeJavaScript("console.log('[Sample Plugin] Hello from context menu')"); } catch {}
+      }
+    });
+  });
+};

plugins/sample-hello/plugin.json

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+{
+  "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
+}

plugins/sample-hello/renderer-preload.js

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+// 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))
+});