feat: ST-201-ST-210 M2 stories, drag-and-drop en Zustand stores

- usePlannerStore met pbiOrder/storyOrder init/reorder/rollback (ST-201)
- useSelectionStore uitgebreid met selectedStoryId en clearSelection (ST-202)
- PBI drag-and-drop binnen prioriteitsgroep via dnd-kit (ST-203)
- PBI slepen over prioriteitsgrens wijzigt priority (ST-204)
- Stories als blokken met prioriteit- en statusbadge (ST-205/ST-206)
- Story drag-and-drop horizontaal binnen en tussen groepen (ST-207)
- Story detail slide-over met bewerkformulier (ST-208)
- Story verwijderen met bevestigingsstap (ST-209)
- Filter op status en prioriteit in rechterpaneel (ST-210)
- Fix: infinite loop in useEffect door stabiele string dependency

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

stores/planner-store.ts

@@ -0,0 +1,46 @@
+import { create } from 'zustand'
+
+interface PlannerStore {
+  // Order maps: productId → pbiId[]
+  pbiOrder: Record<string, string[]>
+  // Order maps: pbiId → storyId[]
+  storyOrder: Record<string, string[]>
+  // Priority maps: pbiId → priority
+  pbiPriority: Record<string, number>
+
+  initPbis: (productId: string, ids: string[]) => void
+  reorderPbis: (productId: string, ids: string[]) => void
+  rollbackPbis: (productId: string, ids: string[]) => void
+  updatePbiPriority: (pbiId: string, priority: number) => void
+
+  initStories: (pbiId: string, ids: string[]) => void
+  reorderStories: (pbiId: string, ids: string[]) => void
+  rollbackStories: (pbiId: string, ids: string[]) => void
+}
+
+export const usePlannerStore = create<PlannerStore>((set) => ({
+  pbiOrder: {},
+  storyOrder: {},
+  pbiPriority: {},
+
+  initPbis: (productId, ids) =>
+    set((state) => ({ pbiOrder: { ...state.pbiOrder, [productId]: ids } })),
+
+  reorderPbis: (productId, ids) =>
+    set((state) => ({ pbiOrder: { ...state.pbiOrder, [productId]: ids } })),
+
+  rollbackPbis: (productId, ids) =>
+    set((state) => ({ pbiOrder: { ...state.pbiOrder, [productId]: ids } })),
+
+  updatePbiPriority: (pbiId, priority) =>
+    set((state) => ({ pbiPriority: { ...state.pbiPriority, [pbiId]: priority } })),
+
+  initStories: (pbiId, ids) =>
+    set((state) => ({ storyOrder: { ...state.storyOrder, [pbiId]: ids } })),
+
+  reorderStories: (pbiId, ids) =>
+    set((state) => ({ storyOrder: { ...state.storyOrder, [pbiId]: ids } })),
+
+  rollbackStories: (pbiId, ids) =>
+    set((state) => ({ storyOrder: { ...state.storyOrder, [pbiId]: ids } })),
+}))