Fix intermittent CID embedded image loading in HTML viewer

- Use proper file:/// URL format (three slashes) for local file references
- Add explicit disk flush when saving embedded images and HTML preview files
- Reorder DisplayMimeMessage to generate HTML preview BEFORE accessing BodyParts
  (fixes issue where cached MimeMessage internal state affected MimeVisitor traversal)

Author: Jaben

src/Papercut.UI/AppLayer/HtmlPreviews/HtmlPreviewGeneratorImpl.cs

@@ -44,7 +44,15 @@ public string GetHtmlPreview(MimeMessage? mailMessageEx, string? tempDir = null)
 
         logger.Verbose("Writing HTML Preview file {HtmlFile}", htmlFile);
 
-        File.WriteAllText(htmlFile, htmlPreview, Encoding.Unicode);
+        // Use explicit FileStream with flush to ensure file is fully written to disk
+        // before WebView2 navigates to it (prevents timing issues with CID images)
+        using (var fs = new FileStream(htmlFile, FileMode.Create, FileAccess.Write, FileShare.None))
+        using (var writer = new StreamWriter(fs, Encoding.Unicode))
+        {
+            writer.Write(htmlPreview);
+            writer.Flush();
+            fs.Flush(flushToDisk: true);
+        }
 
         return htmlFile;
     }

src/Papercut.UI/Helpers/HtmlPreviewVisitor.cs

@@ -110,9 +110,11 @@ private string SaveImage(MimePart image, string url)
         {
             using var output = File.Create(path);
             image.Content.DecodeTo(output);
+            output.Flush(flushToDisk: true); // Ensure file is fully written before WebView2 loads it
         }
 
-        return $"file://{path.Replace('\\', '/')}";
+        // Use proper file:/// URL format (three slashes for local files)
+        return $"file:///{path.Replace('\\', '/')}";
     }
 
     private static string? GetExtensionFromMimeType(string mimeType)
@@ -145,10 +147,9 @@ void WriteTagAsIs()
                 AddMetaCompatibleIeEdge(htmlWriter);
 
                 break;
-                    
+
             case HtmlTagId.Image when !ctx.IsEndTag && _stack.Count > 0:
                 LinkImageTag(ctx, htmlWriter);
-
                 break;
             case HtmlTagId.Body when !ctx.IsEndTag:
                 RemoveContextMenuFromBodyTag(ctx, htmlWriter);

src/Papercut.UI/ViewModels/MessageDetailHtmlViewModel.cs

@@ -542,7 +542,8 @@ await coreWebView.AddScriptToExecuteOnDocumentCreatedAsync(@"
                     }
                     else
                     {
-                        coreWebView.Navigate($"file://{file.Replace("/", @"\")}");
+                        // Use proper file:/// URL format (three slashes for local files)
+                        coreWebView.Navigate($"file:///{file.Replace('\\', '/')}");
                     }
                 }
             );

src/Papercut.UI/ViewModels/MessageDetailViewModel.cs

@@ -306,6 +306,11 @@ void DisplayMimeMessage(MimeMessage? mailMessageEx)
         {
             this.HeaderViewModel.Headers = string.Join("\r\n", mailMessageEx.Headers.Select(h => h.ToString()));
 
+            // IMPORTANT: Generate HTML preview FIRST before any other access to BodyParts
+            // This ensures the MimeVisitor sees the full multipart/related structure
+            // Accessing BodyParts can affect the internal state of cached MimeMessage objects
+            this.HtmlViewModel.ShowMessage(mailMessageEx);
+
             var parts = mailMessageEx.BodyParts.OfType<MimePart>().ToList();
             var mainBody = parts.GetMainBodyTextPart();
 
@@ -330,7 +335,6 @@ void DisplayMimeMessage(MimeMessage? mailMessageEx)
 
             if (mainBody != null) {
                 this.IsHtml = mainBody.IsContentHtml();
-                this.HtmlViewModel.ShowMessage(mailMessageEx);
 
                 if (this.IsHtml)
                 {