feat: enforce single-instance app launch

Use Electron's requestSingleInstanceLock() to prevent opening
multiple instances. When a second instance is attempted, the
existing window is restored and focused instead.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/main/index.ts

@@ -16,6 +16,12 @@ import { createDefaultQuickPromptSettings } from '@shared/domain/tooling';
 
 const { app, BrowserWindow } = electron;
 
+// Enforce single instance — quit immediately if another instance already holds the lock.
+const gotTheLock = app.requestSingleInstanceLock();
+if (!gotTheLock) {
+  app.quit();
+}
+
 let mainWindow: BrowserWindowType | undefined;
 let quickPromptWindow: BrowserWindowType | undefined;
 let appService: AryxAppService | undefined;
@@ -109,6 +115,10 @@ async function bootstrap(): Promise<void> {
   autoUpdateService.start();
 }
 
+app.on('second-instance', () => {
+  showAndFocusWindow();
+});
+
 app.whenReady().then(bootstrap);
 
 app.on('window-all-closed', () => {