Use versioned URLs for static assets

Updated `EnvelopeViewer.razor` to use versioned URLs for CSS and JS files via the new `AppVersionService`, enabling cache busting.

Introduced `AppVersionService` to generate versioned URLs based on the application version retrieved from assembly metadata.

Registered `AppVersionService` as a singleton in `Program.cs` for dependency injection.

EnvelopeGenerator.ReceiverUI/Services/AppVersionService.cs

@@ -0,0 +1,26 @@
+namespace EnvelopeGenerator.ReceiverUI.Services;
+
+/// <summary>
+/// Provides application version for cache busting static assets.
+/// Version is automatically incremented on each build via AssemblyVersion.
+/// </summary>
+public class AppVersionService
+{
+    /// <summary>
+    /// Current application version (e.g., "1.0.0.0")
+    /// </summary>
+    public string Version { get; }
+
+    public AppVersionService()
+    {
+        // Get version from assembly metadata
+        Version = typeof(AppVersionService).Assembly.GetName().Version?.ToString() ?? "1.0.0.0";
+    }
+
+    /// <summary>
+    /// Generates versioned URL for static assets (cache busting)
+    /// </summary>
+    /// <param name="path">Asset path (e.g., "css/envelope-viewer.css")</param>
+    /// <returns>Versioned URL (e.g., "css/envelope-viewer.css?v=1.0.0.0")</returns>
+    public string GetVersionedUrl(string path) => $"{path}?v={Version}";
+}