feat(desktop): add React DevTools extension for Electron dev mode

Downloads and loads React DevTools Chrome extension using Electron 41's
session.extensions API directly, avoiding the deprecated session.loadExtension
used by electron-devtools-installer. Extension is cached in userData after
first download. Only loaded when !app.isPackaged.

packages/desktop/src/main.ts

@@ -87,6 +87,8 @@ async function createMainWindow(): Promise<void> {
   });
 
   if (!app.isPackaged) {
+    const { loadReactDevTools } = await import("./features/react-devtools.js");
+    await loadReactDevTools();
     await mainWindow.loadURL(DEV_SERVER_URL);
     mainWindow.webContents.openDevTools({ mode: "detach" });
     return;