feat: per-class autolabel params + frame-scoped class clear (REQ-180, REQ-181)

- REQ-181: class_params {name: {threshold?, iou_threshold?, min_box_frac?}}
  on /preview and /autolabel, per-class override table in both modals;
  empty overrides take the unchanged global path
- REQ-180: x button on each review sidebar class row clears that class on
  the current frame only via bulk-delete, no confirmation
- includes REQ-178 empty date-folder cycle fix (archive_index.py)

frontend/src/pages/ReviewPage.jsx

@@ -392,6 +392,26 @@ export default function ReviewPage({ batchId: rawBatchId, projectId, onProject }
     } catch (exc) { setError(exc.message) }
   }
 
+  async function clearClassInFrame(item) {
+    const doomed = annotations.filter((row) => row.class_id === item.class_id)
+    if (!doomed.length || !frame) return
+    const ids = new Set(doomed.map((row) => row.id))
+    const previous = annotations
+    const previousCount = frame.annotation_count ?? 0
+    setAnnotations((rows) => rows.filter((row) => !ids.has(row.id)))
+    setMarkedIds((rows) => rows.filter((id) => !ids.has(id)))
+    patchFrameLocally(frame.id, { annotation_count: Math.max(0, previousCount - doomed.length) })
+    try {
+      await api.bulkDeleteAnnotations(doomed.map((row) => row.id))
+      setBatch(await api.getBatch(batchId))
+    } catch (exc) {
+      setAnnotations(previous)
+      setMarkedIds((rows) => [...rows, ...doomed.filter((row) => !rows.includes(row.id)).map((row) => row.id)])
+      patchFrameLocally(frame.id, { annotation_count: previousCount })
+      setError(exc.message)
+    }
+  }
+
   return (
     <>
       <div className="page-head">
@@ -602,6 +622,7 @@ export default function ReviewPage({ batchId: rawBatchId, projectId, onProject }
           activeClass={activeClass}
           reclass={reclass}
           clearClassInBatch={clearClassInBatch}
+          clearClassInFrame={clearClassInFrame}
           annotations={annotations}
           selectedId={selectedId}
           setSelectedId={setSelectedId}