fix(desktop): externalize grammy in Vite Electron build

grammy was not in rollupOptions.external, causing the channels IPC
handlers to fail at runtime with 'No handler registered'.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/desktop/vite.config.ts

@@ -38,6 +38,7 @@ export default defineConfig({
                 '@mariozechner/pi-agent-core',
                 '@mariozechner/pi-ai',
                 '@mariozechner/pi-coding-agent',
+                'grammy',
               ],
             },
           },