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/SearchView.vue

@@ -190,8 +190,8 @@
       v-if="playing"
       simple
       :file-uuid="currentVideo.fileUuid"
-      :start-time="currentVideo.startTime"
-      :end-time="currentVideo.endTime"
+      :start-frame="currentVideo.startFrame"
+      :end-frame="currentVideo.endFrame"
       :title="currentVideo.title"
       @close="playing = false"
     />
@@ -423,7 +423,7 @@ function ensureMinDuration(r: any) {
 }
 
 const playing = ref(false)
-const currentVideo = ref({ fileUuid: '', startTime: 0, endTime: 0, startFrame: null as number | null, endFrame: null as number | null, title: '' })
+const currentVideo = ref({ fileUuid: '', startFrame: 0, endFrame: 0, title: '' })
 
 const currentModeLabel = computed(() => modes.find(m => m.value === mode.value)?.label || 'Keyword')