feat: export AvatarURL for the display picture at any size

- AvatarURL returns the avatar thumbnail, the file path when the thumbnail
  fails, or the gravatar URL, the rule of the user payload
- applyAvatar takes avatar_url from it; the payload is unchanged

controllers/api_controller.go

@@ -1283,17 +1283,9 @@ func applyAvatar(ctx context.Context, app *backpack.App, user *models.User, payl
 	}
 	payload["has_avatar"] = true
 	payload["avatar"] = f.DiskName
-	bucket, ok := app.Lookup[*blob.Bucket]()
-	if !ok || bucket == nil {
-		return
+	if url, err := classes.AvatarURL(ctx, app, user, 128); err == nil && url != "" {
+		payload["avatar_url"] = url
 	}
-	url, err := f.Thumb(ctx, bucket, 128, 128, "auto")
-	if err != nil {
-		slog.Default().Error("golem15.user: avatar thumb", "err", err)
-		payload["avatar_url"] = "/" + attach.BlobKey(f.DiskName)
-		return
-	}
-	payload["avatar_url"] = url
 }
 
 func avatarExt(mime string) (string, bool) {