feat(ST-1116): mobile auto-switch tabs + back button in BacklogSplitPane

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

components/backlog/backlog-split-pane.tsx

@@ -0,0 +1,34 @@
+'use client'
+
+import { useState } from 'react'
+import { useSelectionStore } from '@/stores/selection-store'
+import { SplitPane, type SplitPaneProps } from '@/components/split-pane/split-pane'
+
+type Props = Omit<SplitPaneProps, 'activeTab' | 'onActiveTabChange'>
+
+export function BacklogSplitPane(props: Props) {
+  const { selectedPbiId, selectedStoryId } = useSelectionStore()
+  const [activeTab, setActiveTab] = useState(0)
+
+  // React-recommended "derived state from props" pattern: update state during render
+  // instead of useEffect to avoid cascading renders.
+  const [prevPbiId, setPrevPbiId] = useState(selectedPbiId)
+  const [prevStoryId, setPrevStoryId] = useState(selectedStoryId)
+
+  if (selectedStoryId !== prevStoryId) {
+    setPrevStoryId(selectedStoryId)
+    if (selectedStoryId) setActiveTab(2)
+  }
+  if (selectedPbiId !== prevPbiId) {
+    setPrevPbiId(selectedPbiId)
+    if (selectedPbiId && !selectedStoryId) setActiveTab(1)
+  }
+
+  return (
+    <SplitPane
+      {...props}
+      activeTab={activeTab}
+      onActiveTabChange={setActiveTab}
+    />
+  )
+}