Monetization: Polar chosen as billing provider

- Updated ai.md billing decision to reference Polar (polar.sh)
- Updated code comments (PremiumUrl, IsPremium) to reference Polar
- MONETIZATION.md updated locally (gitignored — confidential)

207 tests, 0 warnings.

ViewModels/MainViewModel.cs

@@ -144,14 +144,14 @@ public 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";
-    // Tabled 2026-08-14: the billing provider is not chosen yet (Gumroad/Lemon Squeezy — TASK 23);
+    // Tabled 2026-08-14: billing provider chosen (Polar — polar.sh);
     // the subscription page (premium unlock) lands here once set up. Coffee/thanks URL is live.
     public const string PremiumUrl = "";
     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 (tabled 2026-08-14 — provider undecided, TASK 23).</summary>
+    /// billing URL is set (Polar — polar.sh, pending TASK 10 implementation).</summary>
     public bool IsPremiumAvailable => PremiumUrl.Length > 0;
 
     /// <summary>Full in-app licensing/legal text, loaded on demand from the
@@ -257,7 +257,7 @@ public class MainViewModel : ViewModelBase
     public SocialsConfig? Socials => _socials;
 
     /// <summary>Freemium: 6 slots — YT + 1 other, the rest locked. Premium seam: all 6 open.</summary>
-    private static bool IsPremium => false; // unlock deferred (TASK 23) — seam only
+    private static bool IsPremium => false; // unlock deferred (TASK 10) — seam only
 
     /// <summary>Canvas.Top for the social bar: 0 = top, 1040 = bottom (40px from the 1080 edge).</summary>
     public double SocialBarTop => _socials?.BarPosition == SocialBarPosition.Top ? 0 : 1040;