Add *very* basic login flow

ui/app-window.slint

@@ -1,6 +1,7 @@
 import { AboutSlint, Button, Palette, HorizontalBox, ScrollView } from "std-widgets.slint";
 import { Timeline } from "timeline.slint";
 import { ImageViewer } from "image-viewer.slint";
+import { LoginView } from "login.slint";
 
 import { Global } from "global.slint";
 export { Global }
@@ -34,7 +35,8 @@ export component AppWindow inherits Window {
 
         Header {}
         
-        Timeline {}
+        if !Global.logged-in: LoginView {}
+        if Global.logged-in: Timeline {}
     }
 
     if Global.previewed-image.asset-id != "" : ImageViewer {