feat(M14): 3-pane backlog — generic SplitPane, BacklogStore, SSE realtime, card-grid TaskPanel (#22)

* feat(split-pane): refactor to generic n-pane SplitPane with cookie persistence

New API: panes[], defaultSplit[], cookieKey, tabLabels. Supports arbitrary
number of panes with n-1 draggable dividers and JSON cookie persistence.
Replaces TriplePane; mobile renders tabs.

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

* feat(split-pane): migrate callers to new panes[] API

Backlog page and sprint board now use generic SplitPane.
TriplePane removed; sprint board uses 3-pane with defaultSplit=[28,35,37].

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

* test(split-pane): add unit tests for 2/3-pane, cookie-restore, mobile tabs

Added jsdom + @testing-library/react devDeps for component testing.
7 cases: render, divider count, cookie restore, invalid cookie fallback,
mobile tab render/switch, and no-dividers-on-mobile.

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

* feat(backlog): add BacklogStore Zustand store with applyChange reducer

State: pbis, storiesByPbi, tasksByStory. setInitialData for server
hydration; applyChange(entity, op, data) handles I/U/D for SSE events.

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

* feat(backlog): server-fetch tasks + hydrate BacklogStore on page load

Page now fetches tasks parallel to stories and groups by story_id.
BacklogHydrationWrapper calls setInitialData on mount so the store
is ready for downstream SSE consumers.

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

* feat(backlog): add EmptyPanel shared component, replace inline empty states

EmptyPanel takes title?, message, and optional action with DemoTooltip.
Replaces duplicate inline empty-state markup in pbi-list and story-panel.

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

* feat(backlog): add TaskPanel with sortable rows and TaskDialog wiring

Reads selectedStoryId + tasksByStory from stores. DnD reorder via
reorderTasksAction. Row click → ?editTask, + button → ?newTask&storyId.
DemoTooltip on drag handles and + button.

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

* feat(backlog): wire TaskPanel + TaskDialog into backlog page

3-pane SplitPane [20,45,35]. searchParams for newTask/editTask.
TaskDialog and EditTaskLoader render on ?newTask and ?editTask.

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

* test(backlog): add TaskPanel tests for render states and click handlers

7 cases: no-story empty, no-tasks empty+action, tasks render, + button
router.push, row click router.push, demo disabled button, demo disabled handles.

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

* feat(backlog): migrate PbiList to store-driven via useBacklogStore

Removes pbis prop; reads from useBacklogStore(s => s.pbis) so SSE
updates reflect in real-time without prop drilling.

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

* feat(backlog): migrate StoryPanel to store-driven + selectStory on click

Removes storiesByPbi prop; reads from useBacklogStore. Card click now
dispatches selectStory(id) + shows isSelected highlight. Edit moved to
inline pencil button. page.tsx drops pbis/storiesByPbi props.

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

* test(backlog): add 3-pane integration tests for click-cascade flow

Covers: empty states, PBI→stories, story→tasks, cascade-reset,
isSelected highlight. localStorage mocked for sort-mode persistence.

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

* feat(ST-1115): SSE backlog realtime — endpoint, hook, hydration mount, tests

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

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

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

* docs(ST-1116): update functional-spec (3-pane backlog + mobile) and architecture (backlog SSE + backlog-store)

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

* feat(ST-1117): TaskPanel card-grid — BacklogCard + rectSortingStrategy, tests updated

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

* fix(tests): correct PbiStatusApi type and remove duplicate mock keys

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

---------

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}
+    />
+  )
+}