Feature: High-quality app login experience

### What feature or improvement do you think would benefit the app?

It would not be very difficult to add a clean sign-in experience to Elapsed.
The Lapse API is authenticated to by making a browser call to the `auth/authorize` endpoint, which redirects to login with Lapse.
The Lapse CLI currently does this; it opens a new browser tab with the Lapse log-in window.

However, the Elapsed app's approach should be much more integrated - especially considering that it isn't possible to open a browser directly on mobile for callback.

These are the different platform implementations:
- Built-in webview page (for mobile)
- Open a full webview window (for desktop)

### Requirements

- [x] Ensure WebView2 is supported on all platforms
- [ ] Add surrounding logic that leads up to login process
- [ ] Add integrated login page (mobile)
- [ ] Open new window (with content extended into titlebar) for login page (desktop)
- [ ] Correctly handle callback from webpage