fix(logs): drop millisecond suffix from log timestamp display (#769)

The log viewer's per-line timestamp now renders HH:mm:ss again. The
HH:mm:ss.SSS variant was busier than the actual log content needed and
made the column harder to scan at a glance. The underlying ISO
timestamp from docker logs -t is still preserved on each row, so
download / copy still carries the original precision.

The rAF-based flush from the same area stays in place; that is what
makes lines feel real-time, not the timestamp width.

frontend/src/components/StructuredLogViewer.tsx

@@ -43,8 +43,7 @@ function formatTs(iso: string | null): string {
   const hh = String(d.getHours()).padStart(2, '0');
   const mm = String(d.getMinutes()).padStart(2, '0');
   const ss = String(d.getSeconds()).padStart(2, '0');
-  const ms = String(d.getMilliseconds()).padStart(3, '0');
-  return `${hh}:${mm}:${ss}.${ms}`;
+  return `${hh}:${mm}:${ss}`;
 }
 
 export default function StructuredLogViewer({ stackName }: StructuredLogViewerProps) {