feat: enforce 100% frame-based positioning (no time)

Core Principle: Frame is the ONLY standard

Changes:
- VideoPlayer: Remove startTime/endTime props
- VideoPlayer: Add mandatory startFrame/endFrame check
- VideoPlayer: UI shows F123 (51.2s) format
- All Views: Only set startFrame/endFrame
- All Views: Only pass :start-frame/:end-frame
- Remove all startTime/endTime from currentVideo refs

Time is only used for display (auxiliary).
Frame is the source of truth for all positioning.

src/views/PeopleView.vue

@@ -274,7 +274,7 @@
       </div>
     </div>
 
-    <VideoPlayer v-if="playing" :file-uuid="currentVideo.fileUuid" :start-time="currentVideo.startTime" :end-time="currentVideo.endTime" :all-traces="clusterVideoTraces" :merged-segments="clusterVideoSegments" :title="currentVideo.title" @close="playing = false; clusterVideoTraces = []; clusterVideoSegments = []" />
+    <VideoPlayer v-if="playing" :file-uuid="currentVideo.fileUuid" :start-frame="currentVideo.startFrame" :end-frame="currentVideo.endFrame" :all-traces="clusterVideoTraces" :merged-segments="clusterVideoSegments" :title="currentVideo.title" @close="playing = false; clusterVideoTraces = []; clusterVideoSegments = []" />
 
     <!-- Merge Target Selection Modal -->
     <div v-if="showMergeTargetModal" class="ms-modal-overlay show" @click.self="showMergeTargetModal = false">
@@ -488,7 +488,7 @@ const loading = computed(() => refreshing.value)
 const faces = ref<any[]>([])
 const traces = ref<any[]>([])
 const playing = ref(false)
-const currentVideo = ref({ fileUuid: '', startTime: 0, endTime: 0, title: '' })
+const currentVideo = ref({ fileUuid: '', startFrame: 0, endFrame: 0, title: '' })
 const candidateThumbs = faceThumbsCache
 const activeFrameCard = ref('')
 const activeFrameMode = ref('')
@@ -1118,7 +1118,12 @@ async function assignClusterTrace(trace: any) {
 }
 
 function playTrace(t: any) {
-  currentVideo.value = { fileUuid: t.file_uuid, startTime: t.first_sec, endTime: t.last_sec, title: `Trace F${t.trace_id} - ${formatTime(t.first_sec)}-${formatTime(t.last_sec)}` }
+  currentVideo.value = {
+    fileUuid: t.file_uuid,
+    startFrame: t.first_frame || 0,
+    endFrame: t.last_frame || (t.first_frame || 0) + 30,
+    title: `F${t.first_frame || 0}-F${t.last_frame || 0} (Trace ${t.trace_id})`
+  }
   playing.value = true
 }
 
@@ -1354,21 +1359,20 @@ async function playFaceDetailVideo() {
     const frameTime = frame / fps
     const startTime = Math.max(0, frameTime - 5)
     const endTime = frameTime + 5
-    currentVideo.value = {
+currentVideo.value = {
       fileUuid: c.file_uuid,
-      startTime,
-      endTime,
-      title: `Face #${frame} (${Math.round(frameTime)}s)`
+      startFrame: startTime * fps,
+      endFrame: endTime * fps,
+      title: `F${Math.round(startTime * fps)}-F${Math.round(endTime * fps)} (${c.name})`
     }
     playing.value = true
   } catch (e) {
     console.error('Failed to get file info:', e)
-    const frameTime = frame / 30
     currentVideo.value = {
       fileUuid: c.file_uuid,
-      startTime: Math.max(0, frameTime - 5),
-      endTime: frameTime + 5,
-      title: `Face #${frame}`
+      startFrame: Math.round(Math.max(0, frameTime - 5) * 30),
+      endFrame: Math.round((frameTime + 5) * 30),
+      title: `F${Math.round(Math.max(0, frameTime - 5) * 30)}-F${Math.round((frameTime + 5) * 30)}`
     }
     playing.value = true
   }