A picture picked is the form's at once, not once its preview is made

Settings' profile kept a picked avatar or header only after the browser had made its small preview, so a Save clicked
meanwhile saved the profile without it and said nothing; the composer added a picture to the post only after its
thumbnail, so a Publish meanwhile left it out. Both now take the file (and the composer starts its upload) as soon as
it is picked, and fill the preview in afterwards; a preview the browser can't make no longer drops the picture,
PrivaPub decides. Found by the e2e suite under load.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LsXgEaXee4GCU1hwYgPJXw

Components/Composer.razor

@@ -518,12 +518,13 @@
 				continue;
 			}
 
+			// the post's at once, and uploading (a publish meanwhile waits for it); its small picture follows
 			var media = new UploadMedia { File = file, FileName = file.Name, ContentType = type, Size = file.Size };
-			if (type.StartsWith("image/"))
-				media.Preview = await Thumbnail(file);
 			form.Media.Add(media);
 			altTextConfirmed = false;
 			Start(media);
+			if (type.StartsWith("image/"))
+				media.Preview = await Thumbnail(file);
 		}
 	}