fix(top bar): a signed-out ON-AIR press starts sign-in instead of disabling the button

Reported: EDIT on + ON-AIR on, signed out → the button reads "Test Stream" and cannot be pressed.
Cause: the primary button bound straight to StartTestCommand, whose CanExecute needs a connection.
StartSession already routed signed-out ON-AIR to the sign-in flow; the test path bypassed it.

The route is now explicit and pure: RouteFor(kind, connected) → SignIn | Record | Live | Test. The
primary command calls SignInAsync for the SignIn route, BeginTestStream for Test, StartSession
otherwise — so the label, the action and the sign-in fallback cannot drift apart. Pinned in
SessionStartDecisionTests for both live and test, connected and not.

ViewModels/MainViewModel.Streaming.cs

@@ -188,11 +188,30 @@ public partial class MainViewModel : ViewModelBase
 
     public string PrimaryStartButtonLabel => SessionStartDecision.LabelFor(PrimaryStartKind);
 
-    /// <summary>The button's action follows its label: Test runs the same go-live pipeline with
-    /// `IsTestStream` set, Go Live runs it for real.</summary>
-    public ICommand PrimaryStartCommand => PrimaryStartKind == SessionStartKind.Test
-        ? StartTestCommand
-        : StartStreamCommand;
+    private ICommand? _primaryStartCommand;
+
+    /// <summary>The button's action follows its label — and a signed-out ON-AIR press starts the
+    /// SIGN-IN flow instead of disabling the control (which is what binding straight to
+    /// <see cref="StartTestCommand"/> did: its CanExecute needs a connection, so the button the
+    /// creator was told to press could not be pressed).</summary>
+    public ICommand PrimaryStartCommand =>
+        _primaryStartCommand ??= new RelayCommand(_ => StartFromPrimaryButton(), _ => ShowPrimaryStartButton);
+
+    private void StartFromPrimaryButton()
+    {
+        switch (SessionStartDecision.RouteFor(PrimaryStartKind, IsConnected))
+        {
+            case SessionStartDecision.Route.SignIn:
+                _ = SignInAsync();
+                return;
+            case SessionStartDecision.Route.Test:
+                BeginTestStream();
+                return;
+            default:
+                StartSession();
+                return;
+        }
+    }
 
     /// <summary>The End button shows while recording OR live.</summary>
     public bool ShowEndStreamButton => IsLive || IsRecording;