fix: enforce frame-based positioning across all video playback

Core Principle: Frame is the only standard

Changes:
- VideoPlayer: Add startFrame/endFrame props
- VideoPlayer: Use frame parameters in simple playback path
- SearchView: Remove frame/time conversion (stop using fps = 24)
- PeopleView: Pass frames directly instead of time
- PersonDetailView: Pass frames directly instead of time

All video streams now use startFrame/endFrame API parameters.
Time conversion only happens in VideoPlayer internals for UI display.

Ref: frame-based positioning standards research

src/components/VideoPlayer.vue

@@ -161,6 +161,8 @@ const props = withDefaults(defineProps<{
   fileUuid: string
   startTime?: number
   endTime?: number
+  startFrame?: number
+  endFrame?: number
   allTraces?: any[]
   mergedSegments?: any[]
   initialTraceIdx?: number
@@ -886,10 +888,15 @@ onMounted(async () => {
     emit('trace-change', currentTraceIdx.value)
   } else {
     try {
-      const st = props.startTime ?? 0
-      pendingSeekTime.value = st
+      const stFrame = props.startFrame ?? 0
+      const enFrame = props.endFrame
+      
       const data = await apiCall('get_video_stream', {
         uuid: props.fileUuid,
+        startTime: null,
+        endTime: null,
+        startFrame: stFrame,
+        endFrame: enFrame,
         original: props.useOriginal,
       })
       if (typeof data === 'string') {