feat(ui): route locked tiers through About

Keep non-owner resolution choices visible but muted. Selecting one preserves the free tier and opens About; external navigation is only the explicit llamacasty.com link. Ref: https://streamlabs.com/desktop

ViewModels/MainViewModel.Overlays.cs

@@ -25,12 +25,11 @@ public partial class MainViewModel : ViewModelBase
     // integration test can assert they point at real destinations.
     public const string ChannelUrl = "https://youtube.com/@llamachileshop";
     public const string MastodonUrl = "https://mastodon.llamachile.tube/@gramps";
-    public const string PremiumUrl = "https://buy.polar.sh/polar_cl_ueMy8AyAClO0o0yo19IroZwKTbD587gzUsjEW3bFapw";
+    public const string PremiumUrl = "https://llamacasty.com";
     public const string CoffeeUrl = "https://buymeacoffee.com/llamachiley";
     public static string AppVersionLabel => $"Version {typeof(MainViewModel).Assembly.GetName().Version?.ToString(3)}";
 
-    /// <summary>The About hub's Unlock Premium button lights up only once the
-    /// billing URL is set (Polar — polar.sh, pending TASK 10 implementation).</summary>
+    /// <summary>The About hub's optional product-information link.</summary>
     public bool IsPremiumAvailable => PremiumUrl.Length > 0;
 
     /// <summary>Full in-app licensing/legal text, loaded on demand from the
@@ -176,6 +175,13 @@ public partial class MainViewModel : ViewModelBase
             IsLicensesOpen = false;
     }
 
+    private void ShowAbout()
+    {
+        IsLicensesOpen = false;
+        IsLicenseEntryOpen = false;
+        ShowOverlay(nameof(IsAboutOpen), "About");
+    }
+
     /// <summary>Shows the in-app licensing panel, loading the full texts from
     /// the shipped THIRD-PARTY-NOTICES.txt on first open (never the OS viewer).
     /// Falls back to a "not found" note if the file is missing so the panel