The earlier commit removed the client_secret and wired
MainActivity.OnNewIntent to AndroidOidcCallbackSink, but
IdentityModel.OidcClient.LoginAsync still had no IBrowser to drive
the user-agent half of the flow. Without it, the desktop / browser
projects continue to fail at login with 'No browser is available'.
Android now plugs in Chrome Custom Tabs:
* PostIt.Android/Services/AndroidSystemBrowser.cs implements
IBrowser.InvokeAsync using CustomTabsIntent.LaunchUrl and waits
for MainActivity.AndroidOidcCallbackSink to deliver the deep-link
Intent (android://postit-signin?code=...&state=...).
* PostIt/Services/Platform.cs is a tiny static indirection the
shared library uses to ask the running platform for an
IBrowser and the appropriate default RedirectUri, without
referencing any UI framework from the shared assembly.
* LoginPageViewModel reads Platform.DefaultRedirectUri and
Platform.CreateBrowser().Invoke() before calling LoginAsync.
* PostIt.Android/PlatformBootstrap.cs wires the Android side at
startup, and MainActivity.OnCreate calls EnsureInitialized().
* Xamarin.AndroidX.Browser 1.8.0 added to the central package
versions so CustomTabsIntent resolves.
