Improve Mastodon image upload failure handling

When image uploads to Mastodon fail (e.g., due to API errors or timeouts),
convert markdown image syntax ![alt](url) to a more readable "alt: url" format
instead of leaving raw markdown code in the toot.

This makes failed image uploads more user-friendly while still preserving
the image URL for users to click and view.

Co-Authored-By: Claudia Sonnet 4.5 <noreply@anthropic.com>

Author: davidwei

src/HappyNotes.Services/MastodonTootService.cs

@@ -296,6 +296,7 @@ private async Task<Status> _EditLongTootAsPhotoAsync(
         }
 
         var successfulUploads = new List<(int index, Attachment attachment)>();
+        var failedIndexes = new HashSet<int>();
 
         try
         {
@@ -315,6 +316,7 @@ private async Task<Status> _EditLongTootAsPhotoAsync(
                 }
                 else if (task.Status == TaskStatus.Faulted)
                 {
+                    failedIndexes.Add(i);
                     logger.LogError(task.Exception, $"Failed to upload image: {matches[i].ImgUrl}");
                 }
             }
@@ -359,6 +361,23 @@ private async Task<Status> _EditLongTootAsPhotoAsync(
             );
         }
 
+        // Replace failed image uploads with "alt: url" format for readability
+        foreach (var failedIndex in failedIndexes)
+        {
+            var img = matches[failedIndex];
+            var altText = img.Alt;
+            altText = altText.Equals("image") ? string.Empty : altText;
+
+            var fallbackText = !string.IsNullOrWhiteSpace(altText)
+                ? $"{altText}: {img.ImgUrl}"
+                : img.ImgUrl;
+
+            markdownText = markdownText.Replace(
+                img.Match,
+                fallbackText
+            );
+        }
+
         return (markdownText.RemoveImageReference(), mediaIds);
     }