feat(backdrop): Capture Window… in-app window pin (TASK 38, Good Dog ONE test)

Click-to-pin an open window as the full-bleed Live backdrop, via the existing
window:<hwnd> capture path (same render layer as game/desktop). Session-scoped:
window:/picker: keys heal to auto on reload (HWND-recycle hazard); pin wins
while the window enumerates, then auto-fallbacks (game → desktop → static); a
dead window's capture session heals to auto, no dead ends.

Fix: ScreenCaptureSourceFactory.Resolve hex-parsed "window:0x<Hwnd>" WITHOUT
stripping the 0x prefix — NumberStyles.HexNumber rejects it, so every pin
resolved null ("No capture target for this key") and the heal silently rolled
back. Same flaw in IsAliveWindowPin (pin-wins guard was dead). Both fixed.

Commit also carries an out-of-scope prerequisite: PillRadioTests.cs:105 had a
committed stray token ("...PrimaryStartButtonLabel soil;", CS1003) blocking the
whole test-project compile — token removed.

Plus the previous session's uncommitted pricing docs (one-time $29/$49) that
share TASKS.md/ai.md/HANDOFF.md.

ViewModels/MainViewModel.cs

@@ -193,6 +193,14 @@ public partial class MainViewModel : ViewModelBase
         ChangeCaptureCommand = new RelayCommand(_ => _ = ChangeBackgroundCaptureAsync());
         RefreshCaptureCommand = new RelayCommand(_ => RefreshBackgroundAutoCapture());
         SetBackgroundDisplayCommand = new RelayCommand(display => SetBackgroundCapture(display as DisplayInfo));
+        _windowEnumerator = WindowEnumeratorOverride ?? new Win32WindowEnumerator();
+        RefreshWindowsCommand = new RelayCommand(_ => RefreshOpenWindows());
+        CaptureWindowCommand = new RelayCommand(window =>
+        {
+            if (window is not WindowInfo info) return;
+            _ = RedesignateBackgroundAsync(info.CaptureKey);
+        });
+        RefreshOpenWindows();
         ToggleShowDesktopCommand = new RelayCommand(_ => ToggleBackgroundShowDesktop());
         BrowseBackgroundCommand = new RelayCommand(_ => BrowseBackground());
         ToggleMicMuteCommand = new RelayCommand(_ => ToggleMicMute());
@@ -285,8 +293,7 @@ public partial class MainViewModel : ViewModelBase
             _screenCaptureFactory.Resolve,
             System.Windows.Application.Current?.Dispatcher);
         _screenCaptureManager.PreviewBitmapChanged += OnScreenPreviewBitmapChanged;
-        _screenCaptureManager.CaptureFailed += (key, message) =>
-            AppLog.Write($"ScreenCaptureManager: capture '{key}' failed: {message}");
+        _screenCaptureManager.CaptureFailed += OnScreenCaptureFailed;
 
         _mediaManager = new MediaVideoSourceManager(
             path => new MediaVideoSource(