Add SPA fallback support to YARP container image

When YARP_ENABLE_STATIC_FILES is enabled, MapFallbackToFile("index.html")
is registered after MapReverseProxy() so unmatched requests serve index.html
for client-side routing. Set YARP_DISABLE_SPA_FALLBACK=true to opt out.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: davidfowl

src/Application/Program.cs

@@ -39,6 +39,15 @@
 app.UseRouting();
 app.MapReverseProxy();
 
+if (string.Equals(isEnabledStaticFiles, "true", StringComparison.OrdinalIgnoreCase))
+{
+    var disableSpaFallback = Environment.GetEnvironmentVariable("YARP_DISABLE_SPA_FALLBACK");
+    if (!string.Equals(disableSpaFallback, "true", StringComparison.OrdinalIgnoreCase))
+    {
+        app.MapFallbackToFile("index.html");
+    }
+}
+
 await app.RunAsync();
 
 return 0;