fix: handle missing frame params in simple playback path

Problem: LibraryView calls VideoPlayer without frame params, causing:
- startFrame=0, endFrame=undefined passed to API
- Backend returns error or malformed stream
- Video playback error: MEDIA_ELEMENT_ERROR: Format error

Solution: Only pass frame params when both startFrame and endFrame are defined.
If missing, play full video (no frame params to API).

Fixes regression from commit 1fd6312 (frame-based positioning)

src/components/VideoPlayer.vue

@@ -888,17 +888,17 @@ onMounted(async () => {
     emit('trace-change', currentTraceIdx.value)
   } else {
     try {
-      const stFrame = props.startFrame ?? 0
-      const enFrame = props.endFrame
-      
-      const data = await apiCall('get_video_stream', {
+      const params: any = {
         uuid: props.fileUuid,
-        startTime: null,
-        endTime: null,
-        startFrame: stFrame,
-        endFrame: enFrame,
         original: props.useOriginal,
-      })
+      }
+      
+      if (props.startFrame != null && props.endFrame != null) {
+        params.startFrame = props.startFrame
+        params.endFrame = props.endFrame
+      }
+      
+      const data = await apiCall('get_video_stream', params)
       if (typeof data === 'string') {
         videoSrc.value = data
       } else {