fix: track 5 frames works for bbox and polygon shapes (REQ-189)

trackForward read geometry.coordinates, a key the backend never emits, and
had no bbox branch - the action silently no-op'd for every shape (flagged
dead in docs/audit-2026-08-07.md:103).

- bbox points used as-is, polygon reduced to its bounding box
- guards speak instead of returning silently
- a frame SAM3 refuses no longer aborts the remaining frames
- a press during an in-flight run is ignored (would double-write)
- REQ-189 added; non-goal requirements.md:22 amended to keep exemplar
  propagation out while admitting this one-shot hand-off

frontend/src/pages/ReviewPage.jsx

@@ -355,15 +355,27 @@ export default function ReviewPage({ batchId: rawBatchId, projectId, onProject }
   }, [frames, index, frame, hiddenClasses])
 
   const trackForward = useCallback(async () => {
-    if (selectedId == null || !frames?.length) return
+    if (busy) return
+    if (!frames?.length) return
+    if (selectedId == null) {
+      setError('Select a shape first — Track 5 Frames follows one shape forward.')
+      return
+    }
     const current = annotations.find((a) => a.id === selectedId)
-    if (!current) return
-    
+    if (!current) {
+      setError('That shape is gone — select another one.')
+      return
+    }
+
+    // SAM3 assist wants one normalized [x0, y0, x1, y1] box (REQ-043). A bbox
+    // shape already is that; a polygon is reduced to its bounding box.
+    const geometry = current.geometry
     let box = null
-    if (current.geometry?.type === 'polygon' && current.geometry.coordinates?.[0]) {
-      const pts = current.geometry.coordinates[0]
+    if (geometry?.type === 'bbox' && geometry.points?.length === 4) {
+      box = geometry.points
+    } else if (geometry?.type === 'polygon' && geometry.points?.length) {
       let minX = 1, minY = 1, maxX = 0, maxY = 0
-      for (const [x, y] of pts) {
+      for (const [x, y] of geometry.points) {
         if (x < minX) minX = x
         if (x > maxX) maxX = x
         if (y < minY) minY = y
@@ -371,27 +383,38 @@ export default function ReviewPage({ batchId: rawBatchId, projectId, onProject }
       }
       box = [minX, minY, maxX, maxY]
     }
-    if (!box) return
+    if (!box) {
+      setError('That shape has no usable box — Track 5 Frames needs one.')
+      return
+    }
+    const ahead = frames.slice(index + 1, index + 6)
+    if (!ahead.length) {
+      setError('Last frame — Track 5 Frames has no frames ahead to follow into.')
+      return
+    }
 
     setBusy(true)
     setError('')
     try {
-      let updatedCount = 0
-      for (let i = 1; i <= 5; i++) {
-        const nextIdx = index + i
-        if (nextIdx >= frames.length) break
-        const targetFrame = frames[nextIdx]
-        
-        await api.assist(targetFrame.id, { box, class_id: current.class_id })
-        updatedCount++
-        patchFrameLocally(targetFrame.id, { annotation_count: (targetFrame.annotation_count ?? 0) + 1 })
+      let tracked = 0
+      const failures = []
+      for (let i = 0; i < ahead.length; i++) {
+        const targetFrame = ahead[i]
+        try {
+          await api.assist(targetFrame.id, { box, class_id: current.class_id })
+          tracked++
+          patchFrameLocally(targetFrame.id, { annotation_count: (targetFrame.annotation_count ?? 0) + 1 })
+        } catch (exc) {
+          failures.push(`frame ${index + i + 2}: ${exc.message}`)
+        }
+      }
+      if (failures.length) {
+        setError(`Tracked ${tracked} of ${ahead.length} — ${failures.join(' · ')}`)
       }
-    } catch (exc) {
-      setError(exc.message)
     } finally {
       setBusy(false)
     }
-  }, [annotations, selectedId, index, frames])
+  }, [annotations, selectedId, index, frames, busy])
 
   const stateRef = useRef({})
   stateRef.current = { frames, index, project, selectedId, setStatus, removeSelected, reclass, jumpToPending, jumpToNextAnnotated, setAssistMode, copyPrevious, trackForward, mode, setMode, markedIds, removeMarked, reclassMarked, setMarkedIds, toggleHideSelected }