PostIt is a desktop/mobile app talking to Yavsc.Org
(https://yavsc.pschneider.fr) as an OIDC identity provider. The
previous grant used the client_credentials flow with a client_secret
embedded in postit-settings.json: this was both insecure (secret
travels with the binary) and inappropriate for an interactive app
(token had no user identity, so the API could not scope or audit).
The new flow is Authorization Code + PKCE:
* PostIt client (Settings/AuthenticationSettings.cs): the
ClientSecret property is removed; GetOidcClientOptions now drops
the secret and accepts an optional IBrowser supplied per-platform.
* Settings.cs: new AndroidRedirectUri constant ('android://postit-signin')
that the Android app uses; RedirectUri is no longer hard-coded in
MainViewModel.
* MainViewModel.cs: the manual discovery + client_credentials POST is
replaced with OidcClient.LoginAsync (Authorization Code + PKCE).
* Settings sample: Authority points at the real Yavsc.Org OP, not at
a non-existent Keycloak-style realm path.
* Yavsc.Org/Extensions/HostingExtensions.cs: the 'postit' client seed
is now idempotent (MigratePostItClientToPublic) and detects
legacy state on existing ConfigurationDb rows - flips
RequireClientSecret=false, RequirePkce=true, drops any ClientSecret
row, and replaces the legacy RedirectUris
(https://yavsc.pschneider.fr/, yavsc://callback) with the current
set (http://127.0.0.1:7890/, android://postit-signin).
PostIt.Android:
* MainActivity: explicit Name attribute so the activity alias can
target a stable component; LaunchMode.SingleTask so the existing
instance receives the deep-link Intent; OnNewIntent forwards the
callback URI through AndroidOidcCallbackSink.
* AndroidManifest.xml: activity-alias PostIt.Android.OidcCallbackActivity
exposing scheme=android host=postit-signin to Android, so the OP
redirect lands back in the running PostIt instance.
The IdentityModel.OidcClient.Browser.SystemBrowser package and a
thin AndroidSystemBrowser implementation are added in a follow-up so
OidcClient.LoginAsync can actually drive Chrome Custom Tabs and
consume AndroidOidcCallbackSink.
