Image viewer: construct ImageSource from bitmap to bypass Task binding race

The viewer's template selector binds to ImageSource.TemplateKey via a
Task-based async binding (TemplateKeyAsync^). On the first read the
Task is pending and the binding returns default(ImageSourceTemplateType)
= Default, which is mapped to a "Unsupported Format" text template.
The selector doesn't re-pick when the Task completes later, so the
"unsupported" text sticks around even though TemplateKey eventually
resolved to Image.

The previous fix (await GetOrRefreshTemplateKeyAsync before assigning)
didn't help because TemplateKeyAsync returns a *fresh* Task on every
access — pre-resolving the underlying field still goes through a new
Task at binding time.

Switch to ImageSource's bitmap-only constructor, which sets
TemplateKey = Image synchronously in the ctor body. AdvancedImageBox
renders the decoded bitmap regardless of whether the bytes were JPEG,
PNG, or WebP, which is exactly what we need for CivArchive's
extensionless CDN URLs (img.genur.art/sig/.../base64).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: mohnjiles

StabilityMatrix.Avalonia/ViewModels/CheckpointBrowser/CivArchiveDetailsPageViewModel.cs

@@ -462,19 +462,22 @@ private async Task ShowImageDialog(CivArchiveModelImage? image)
     }
 
     /// <summary>
-    /// Build an <see cref="ImageSource"/> ready for the image viewer to render. Critically,
-    /// <see cref="ImageSource.GetOrRefreshTemplateKeyAsync"/> must run before assigning so the
-    /// viewer's template selector picks the right control for animated WebPs vs static images
-    /// — otherwise navigation to certain images shows "Unsupported format".
+    /// Build an <see cref="ImageSource"/> ready for the image viewer to render.
+    /// The viewer's template selector keys off <c>ImageSource.TemplateKey</c>; if that's
+    /// <c>Default</c>, the selector renders the literal "Unsupported Format" text. The
+    /// URL-construction path leaves TemplateKey as Default until a Task-based binding
+    /// resolves it, which races with the viewer's first paint on extensionless CivArchive
+    /// CDN URLs (e.g. <c>img.genur.art/sig/.../base64</c>). Use the bitmap-only constructor
+    /// instead — it sets TemplateKey to Image synchronously, which the AdvancedImageBox
+    /// can render whether the bytes were JPEG, PNG, or WebP.
     /// </summary>
     private static async Task<ImageSource?> PrepareImageSourceAsync(string url)
     {
         try
         {
-            var source = new ImageSource(new Uri(url));
-            await source.GetBitmapAsync();
-            await source.GetOrRefreshTemplateKeyAsync();
-            return source;
+            var loader = new ImageSource(new Uri(url));
+            var bitmap = await loader.GetBitmapAsync();
+            return bitmap is not null ? new ImageSource(bitmap) { RemoteUrl = new Uri(url) } : null;
         }
         catch
         {