send the map selection to change detection and show the modified class

The comparison hook runs before the selection state is declared in App, so the
selection is read through a getter at the moment the run starts rather than
captured at render. The panel gains the modified count, which until now was
folded into removed plus added.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

frontend/src/hooks/useChangeDetectionWorkflow.ts

@@ -1,18 +1,29 @@
 import { useState } from 'react'
 import { analysisApi } from '../services/api'
-import type { ChangeDetectionSummary, DatasetCreateResponse } from '../types'
+import type { ChangeDetectionSummary, DatasetCreateResponse, VectorSelectionBBox } from '../types'
 import { formatError } from '../lib/formatError'
 
 interface ChangeDetectionWorkflowOptions {
   selectedProjectId: string | null
   availableVectorDatasets: DatasetCreateResponse[]
   loadDatasetJobs: (projectId: string, datasetId: string) => Promise<void>
+  /**
+   * Reads the active map selection when the comparison is started. Without a
+   * selection the comparison covers both datasets entire, which is rarely the
+   * question being asked and returns a payload no map can draw.
+   *
+   * A getter rather than a value because this hook runs before the selection
+   * state is declared in the component; it is only ever called from a user
+   * event, long after that binding is initialised.
+   */
+  getSelection?: () => { bbox: VectorSelectionBBox | null; areaId: string | null }
 }
 
 export function useChangeDetectionWorkflow({
   selectedProjectId,
   availableVectorDatasets,
   loadDatasetJobs,
+  getSelection,
 }: ChangeDetectionWorkflowOptions) {
   const [changeSourceDatasetId, setChangeSourceDatasetId] = useState('')
   const [changeTargetDatasetId, setChangeTargetDatasetId] = useState('')
@@ -41,12 +52,15 @@ export function useChangeDetectionWorkflow({
     setChangeDetectionError(null)
     setChangeDetectionResult(null)
     setRunningChangeDetection(true)
+    const selection = getSelection?.() ?? { bbox: null, areaId: null }
     try {
       const job = await analysisApi.runChangeDetection({
         source_dataset_id: sourceDatasetId,
         target_dataset_id: targetDatasetId,
         iou_threshold: changeIouThreshold,
         include_unchanged: changeIncludeUnchanged,
+        ...(selection.bbox ? { bbox: selection.bbox } : {}),
+        ...(selection.areaId ? { area_id: selection.areaId } : {}),
       })
       if (job.status !== 'success') {
         throw new Error(job.error_message || 'Change detection job failed')