feat: per-class max box fraction (REQ-188)

- labeling/preview/autolabel: max_box_frac + per-class max_box_fracs
  ceiling filter (0=none, 1=off) before NMS, mirrors min_box_frac
- exemplar review-assist: ceiling before max_detections truncation;
  ExemplarLabelRequest + filter panel 'Max box size' slider
- ClassParamsTable: MaxBox column; copy line now REQ-186 amended
  order: class <name> conf <v> iou <v> minbox <v> maxbox <v> container
- docs: design/ui-spec/tasks updated (incl. stale 4-slider narrative)

backend/api/review.py

@@ -56,6 +56,7 @@ class ExemplarLabelRequest(BaseModel):
     threshold: float = 0.5
     iou_threshold: float = 0.8
     min_box_frac: float = 0.002
+    max_box_frac: float = 1.0
     max_detections: int = 100
     # Off by default: a drag previews, only Apply writes.
     apply: bool = False
@@ -131,6 +132,7 @@ def exemplar_label(frame_id: int, request: ExemplarLabelRequest) -> dict:
             threshold=request.threshold,
             iou_threshold=request.iou_threshold,
             min_box_frac=request.min_box_frac,
+            max_box_frac=request.max_box_frac,
             max_detections=request.max_detections,
             apply=request.apply,
         )