Add an Electron app drag handle to sign-in page (#3795)

* Add Electron app drag handle to sign-in page

* Don't assign drag regions in web from JSX, results in dev-only console errors about unsupported style values

src/components/AppHeader.module.css

@@ -6,6 +6,9 @@
   grid-template-columns: 1fr auto 1fr;
   user-select: none;
   -webkit-user-select: none;
+}
+
+.header.desktopApp {
   /* Make the header act as a handle to drag the electron app window,
    * per the electron docs: https://www.electronjs.org/docs/latest/tutorial/window-customization#set-custom-draggable-region
    * all interactive elements opt-out of this behavior by default in src/index.css