PostIt: drop LoginPage, hoist Login into session banner, drop AuthorId field

Login flow no longer needs a dedicated page. The OIDC interactive
login now lives on the persistent SessionStatusBanner, alongside
'Se déconnecter', driven by a new SessionStatusViewModel.LoginAsync
command. On success the VM raises LoginSucceeded and App.axaml.cs
pushes MainPage on top of HomePage — same path BootAsync already
takes when the silent refresh succeeds at boot, so the two flows
can't drift apart (PushMainPageAsync helper, single source of
truth).

MainPage no longer shows an editable AuthorId field: the server
infers the author from the bearer token, so the client-side
control was misleading at best. The detail grid drops from 5 rows
to 4.

Removed:
  - Views/LoginPage.axaml + .axaml.cs
  - ViewModels/LoginPageViewModel.cs
  - HomePage Login button + OnLoginClick code-behind
  - DI registrations for LoginPage / LoginPageViewModel
  - ViewLocator mapping
  - dangling <c>LoginPage*</c> cref / comments in Platform.cs,
    PlatformBootstrap.cs (Desktop + Android), MainWindow.axaml,
    YavscApiClient.cs

src/PostIt/PostIt.Desktop/PlatformBootstrap.cs

@@ -5,8 +5,8 @@ namespace PostIt.Desktop;
 
 /// <summary>
 /// One-shot platform bootstrap. Called from <c>Program.Main</c> so that
-/// the shared <c>LoginPageViewModel</c> sees a working <c>IBrowser</c>
-/// — the custom-scheme browser that hands the OIDC callback off to the
+/// the shared OIDC login path sees a working <c>IBrowser</c> — the
+/// custom-scheme browser that hands the OIDC callback off to the
 /// running instance through the named pipe. Desktop builds do NOT use
 /// a loopback HTTP listener: the <c>postit://</c> scheme is registered
 /// with the OS at install time and the browser is whatever the user