Fix TypeScript build errors

- Remove unused Event import from client.ts
- Fix fractionalSecondDigits incompatibility in EventCard.tsx
- Manually format milliseconds for timestamp display

frontend/src/api/client.ts

@@ -1,7 +1,7 @@
 // HTTP API client using axios
 
 import axios from 'axios';
-import type { Event, Statistics, EventListResponse } from '../types';
+import type { Statistics, EventListResponse } from '../types';
 
 const API_BASE_URL = import.meta.env.VITE_API_URL || 'http://localhost:8000';
 

frontend/src/components/EventCard.tsx

@@ -9,13 +9,14 @@ interface EventCardProps {
 
 const formatTimestamp = (timestamp: number): string => {
   const date = new Date(timestamp * 1000);
-  return date.toLocaleTimeString('en-US', {
+  const timeString = date.toLocaleTimeString('en-US', {
     hour: '2-digit',
     minute: '2-digit',
     second: '2-digit',
-    fractionalSecondDigits: 3,
     hour12: false,
   });
+  const ms = Math.floor((timestamp % 1) * 1000).toString().padStart(3, '0');
+  return `${timeString}.${ms}`;
 };
 
 const getEventColor = (eventType: string): string => {