perf: use direct trace-thumb endpoint + fallback for face group thumbnails

- loadTraceThumb now uses /api/v1/trace-thumb/{uuid}/{tid} (~8ms)
- Face group imgs use direct trace-thumb URL with fallback
- Merge dialogs also use direct trace-thumb with fallback
- MAX_CONCURRENT increased to 32

Fallback chain:
1. c.rep_trace.file_uuid/trace_id (full trace data)
2. selectedFileUuid/representative_trace (group data only)
3. SVG silhouette

src/store.ts

@@ -332,7 +332,7 @@ loadProgressCacheFromStorage()
 
 const thumbQueue: (() => Promise<void>)[] = []
 let activeThumbLoads = 0
-const MAX_CONCURRENT = 16
+const MAX_CONCURRENT = 32
 
 const profileQueue: (() => Promise<void>)[] = []
 let activeProfileLoads = 0
@@ -592,15 +592,14 @@ export function loadTraceThumb(t: any) {
   loadingFaceThumbs.add(key)
   queueThumb(async () => {
     try {
-      const args: any = { uuid: t.file_uuid, frame: t.best_face_frame || t.start_frame || 0 }
-      if (t.best_face_bbox) {
-        args.bboxX = t.best_face_bbox.x
-        args.bboxY = t.best_face_bbox.y
-        args.bboxW = t.best_face_bbox.width
-        args.bboxH = t.best_face_bbox.height
+      // Use direct trace-thumb endpoint for fast keyFace.jpg loading (~8ms)
+      const url = `/api/v1/trace-thumb/${t.file_uuid}/${t.trace_id}`
+      const resp = await fetch(url)
+      if (resp.ok) {
+        const blob = await resp.blob()
+        const dataUrl = URL.createObjectURL(blob)
+        faceThumbsCache.value[key] = dataUrl
       }
-      const result = await apiCall('get_face_thumbnail', args)
-      if (result) faceThumbsCache.value[key] = result
     } catch (e) {
       console.error('loadTraceThumb failed:', key, e)
     } finally {