Fix deprecated --highlight-style flag in preview-text.ts

Update to use --syntax-highlighting for Pandoc 3.8 compatibility.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: cderv

src/preview/preview-text.ts

@@ -150,7 +150,7 @@ async function textPreviewHtml(file: string, req: Request) {
   const cmd = [pandocBinaryPath()];
   cmd.push("--to", "html");
   cmd.push(
-    "--highlight-style",
+    "--syntax-highlighting",
     textHighlightThemePath("atom-one", darkMode ? "dark" : "light")!,
   );
   cmd.push("--standalone");