The previous commit set Platform.CreateBrowser to null on the desktop
side, so LoginAsync would still fail with 'No browser is available'.
Close that loop with an explicit desktop bootstrap.
PostIt.Desktop/PlatformBootstrap.cs mirrors the Android side: it
populates Platform.DefaultRedirectUri and Platform.CreateBrowser
once at startup. Program.Main calls EnsureInitialized before
BuildAvaloniaApp so the LoginPageViewModel sees a working browser
before any login attempt.
The Yavsc.Org seed now reads Site:ExternalUrl from configuration so
the RedirectUri list for the PostIt client follows the same setting
as the rest of the application (same value used in
Administration/ClientController, AccountController, etc.). Without
this, an embedded 'launch PostIt from a Yavsc.Org page' scenario
would be rejected by IdentityServer (redirect_uri mismatch).
BuildPostItRedirectUris is a small helper that yields the constant
PostItRedirectUris (loopback + Android custom scheme) followed by
Site:ExternalUrl when set. Both SeedNewPostItClient (fresh db) and
MigratePostItClientToPublic (existing db) consume it. The legacy
cleanup block (which used to remove https://yavsc.pschneider.fr/
and yavsc://callback) is dropped: Site:ExternalUrl is now the
canonical way to authorise that path and may legitimately equal
that value.
