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/views/PersonDetailView.vue

@@ -443,8 +443,8 @@ function playTrace(t: any) {
   const endFrame = (t.last_frame || 0) + 10
   currentVideo.value = {
     fileUuid: t.file_uuid || '',
-    startTime: startFrame / fps,
-    endTime: endFrame / fps,
+    startFrame: startFrame,
+    endFrame: endFrame,
     traceIdx: idx,
     title: `${person.value?.name} · T${t.trace_id}`,
   }