fix(frontend): change time format to 12-hour with AM/PM

- Change time locale from zh-CN to en-US to properly display AM/PM
- Increase letter-spacing for better readability (0.08em → 0.12em)
- Update all time displays to use 12-hour format consistently

frontend/src/pages/chat/composables/useClientTime.ts

@@ -36,7 +36,7 @@ export function useClientTime() {
   }
 
   function formatClientClock(date: Date) {
-    return date.toLocaleTimeString('zh-CN', { hour12: false, hour: '2-digit', minute: '2-digit', second: '2-digit' })
+    return date.toLocaleTimeString('en-US', { hour12: true, hour: '2-digit', minute: '2-digit', second: '2-digit' })
   }
 
   function weatherCodeLabel(code: number | null | undefined) {