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/client/components/ExportButton.tsx

@@ -65,7 +65,7 @@ export function ExportButton({
         btn btn-sm flex-shrink-0 gap-1.5 transition-all duration-200
         ${state === "success" ? "text-white shadow-glow-success" : ""}
         ${state === "error" ? "text-white" : ""}
-        ${state === "idle" || state === "exporting" ? "btn-primary" : ""}
+        ${state === "idle" || state === "exporting" ? "btn-secondary" : ""}
       `}
       style={
         state === "success"