fix(welcome): FlavourStep notifies host on select so Next enables immediately

WIP on local branch feat/wizard-recipes (NOT pushed) — holding per operator while
more wizard changes (role app-recipes) are designed.

windows/welcome/src/SilverOS.Welcome.UI/Components/Routes.razor

@@ -37,7 +37,7 @@
                     <WelcomeStep />
                     break;
                 case 1:
-                    <FlavourStep Flavours="_flavours" />
+                    <FlavourStep Flavours="_flavours" OnSelected="StateHasChanged" />
                     break;
                 case 2:
                     <AccountStep OnValidityChanged="@(v => { _accountValid = v; StateHasChanged(); })" />

windows/welcome/src/SilverOS.Welcome.UI/Components/Steps/FlavourStep.razor

@@ -19,13 +19,19 @@
 @code {
     [Parameter] public IReadOnlyList<FlavourManifest> Flavours { get; set; } = Array.Empty<FlavourManifest>();
 
-    protected override void OnInitialized()
+    /// <summary>Notifies the wizard host when the selection changes so it re-evaluates
+    /// the Next button (otherwise Next stays disabled until a back/forward re-render).</summary>
+    [Parameter] public EventCallback OnSelected { get; set; }
+
+    protected override async Task OnInitializedAsync()
     {
         State.Flavour ??= Flavours.FirstOrDefault(f => f.IsDefault);
+        await OnSelected.InvokeAsync();
     }
 
-    void Select(FlavourManifest f)
+    async Task Select(FlavourManifest f)
     {
         State.Flavour = f;
+        await OnSelected.InvokeAsync();
     }
 }