Surface "no download available" when downloadUris is empty

ExecuteDownloadAsync silently returned when the resolved URI list was
empty (every mirror was gated/paid, or the file had no DownloadUrl on
any source). The user saw the button briefly disable while the async
method ran, then re-enable, with zero feedback — matching the report
of "button went grey but nothing happened".

Now shows a clear "No download available" notification explaining why
nothing happened. Split out from the Model-is-null guard since that's
not a user-facing condition.

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

Author: mohnjiles

StabilityMatrix.Avalonia/ViewModels/CheckpointBrowser/CivArchiveDetailsPageViewModel.cs

@@ -663,8 +663,17 @@ private async Task ExecuteDownloadAsync(
         string? sourceLabel
     )
     {
-        if (downloadUris.Count == 0 || Model is null)
+        if (Model is null)
+            return;
+
+        if (downloadUris.Count == 0)
+        {
+            notificationService.Show(
+                "No download available",
+                "This file has no usable download URL — every mirror was either missing or gated/paid."
+            );
             return;
+        }
 
         if (!settingsManager.IsLibraryDirSet)
         {