Fix main routing between /login and /pos

should only show one or the other depending on user login status

wuttapos/app.py

@@ -91,12 +91,30 @@ def main(page: ft.Page):
 
     # cf .https://flet.dev/docs/guides/python/navigation-and-routing#building-views-on-route-change
 
-    def route_change(route):
+    def route_change(e):
         page.views.clear()
+
+        redirect = None
+        user_uuid = page.session.get('user_uuid')
+        if page.route == '/login' and user_uuid:
+            redirect = '/pos'
+            other = '/pos'
+        elif page.route == '/pos' and not user_uuid:
+            redirect = '/login'
+            other = '/login'
+        else:
+            redirect = '/pos' if user_uuid else '/login'
+
+        if redirect and page.route != redirect:
+            page.go(other)
+            return
+
         if page.route == '/pos':
             page.views.append(POSView(config, '/pos'))
+
         elif page.route == '/login':
             page.views.append(LoginView(config, '/login'))
+
         page.update()
 
     # TODO: this was in example docs but not sure what it's for?