feat: Sign in Window views & view models

[spikecurtis]

Adds views for the Sign In dialog window, and a backing view model.

This is still a bit rough around the edges, but I'd like to get some eyes on it and merge to iterate.

It also needs to be integrated with some of the structural changes in #19 including dependency injection. That can happen per or post merge, depending on when #19 lands in main.

Known issues:

• Input validation. If you give an invalid URL, then clicking "Next" will crash the app since the hyperlink button won't accept an invalid URI.
• Relatively janky Error message handling -- doesn't use any consistent styling.

[spikecurtis]

• feat: Sign in Window views & view models #20 👈 (View in Graphite)
• main

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[deansheather]

I think the better way of doing forwards/backwards navigation is to use a <RootFrame> in the window rather than setting Window.Content directly.

One pitfall that might make it hard to use the RootFrame's built-in navigation stuff is that I don't think it lets you instantiate the Window types yourself - which makes it hard to get a ViewModel in there.

In the TrayWindow code in my PR, I ended up just setting RootFrame.Content directly because of this limitation (and because I don't need forwards/backwards navigation). But if you could get that working it'd be more idiomatic I think

[deansheather]

I switched it to using a Frame so we're not just touching the window content directly, but I'm not using the forward/backwards functionality yet because of the aforementioned issue.

In the future if we solve it, we could have the "Coder Desktop" text and the navigation buttons be on the Window itself, rather than in each page.