V15: Show a loader during the login procedures rather than oddly styled content

[iOvergaard]

Description

Fixes #17453

• Shows only a loader for a normal login flow
• Shows the umb-app-error component only if the flow fails (either in the main window or a popup)
• Fixes an error where hasOwnOpener did not work (at least not on the Vite server)

How to test

1. Log in as normal and verify that you do not see a "flash of oddly styled content" but instead a uui-loader (you probably have to slow down the network requests)
2. Try and force a session timeout, e.g. by changing the access_token in Local Storage and performing some request, e.g. going to a content page
3. You should now see the login timeout overlay
4. Verify that the popup is closed after login and that you only see a loader inside the popup

Hint: You can force the "login landing page" by going to https://localhost:44339/umbraco/oauth_complete and appending ?code=test to see the success variant and omitting the ?code parameter to see the failure variant.

[leekelleher]

Tested out, works as expected! 🚀