fix: HTML artifact preview renders blank in preview mode (#876)

The condition guarding ArtifactFilePreview only allowed markdown files
through, which prevented HTML files from reaching the preview component.
Added `language === "html"` to the condition so HTML artifacts render
correctly in preview mode.

Fixes #873

Co-authored-by: Claude <noreply@anthropic.com>

frontend/src/components/workspace/artifacts/artifact-file-detail.tsx

@@ -234,8 +234,7 @@ export function ArtifactFileDetail({
       <ArtifactContent className="p-0">
         {previewable &&
           viewMode === "preview" &&
-          language === "markdown" &&
-          content && (
+          (language === "markdown" || language === "html") && (
             <ArtifactFilePreview
               filepath={filepath}
               threadId={threadId}