Polish: simplify formatTimestamp and tone down export button

Replace try/catch with isNaN guard in the HTML exporter's
formatTimestamp, matching the same cleanup applied client-side.

Downgrade the export button from btn-primary to btn-secondary so it
doesn't compete visually with the main content area. The primary blue
gradient was overly prominent for a utility action.

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

src/server/services/html-exporter.ts

@@ -170,16 +170,13 @@ function renderMarkdown(text: string): string {
 }
 
 function formatTimestamp(ts: string): string {
-  try {
-    const d = new Date(ts);
-    return d.toLocaleTimeString(undefined, {
-      hour: "2-digit",
-      minute: "2-digit",
-      second: "2-digit",
-    });
-  } catch {
-    return "";
-  }
+  const d = new Date(ts);
+  if (isNaN(d.getTime())) return "";
+  return d.toLocaleTimeString(undefined, {
+    hour: "2-digit",
+    minute: "2-digit",
+    second: "2-digit",
+  });
 }
 
 function getHighlightCss(): string {