feat(ST-356): add solo Kanban board with DnD and Zustand store

- useSoloStore: initTasks, optimisticMove (returns prev status), rollback, updatePlan
- SoloBoard: DndContext with PointerSensor (distance:5), closestCorners, 3 columns
- SoloColumn: useDroppable per status, MD3 status-color headers, task count, empty state
- SoloTaskCard + SoloTaskCardOverlay: useDraggable (disabled for demo), priority left-border
- onDragEnd: optimisticMove → updateTaskStatusAction → rollback + toast on error
- REVIEW tasks mapped to IN_PROGRESS column

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

stores/solo-store.ts

@@ -0,0 +1,32 @@
+import { create } from 'zustand'
+import type { SoloTask } from '@/components/solo/solo-board'
+
+type TaskStatus = SoloTask['status']
+
+interface SoloStore {
+  tasks: Record<string, SoloTask>
+  initTasks: (tasks: SoloTask[]) => void
+  optimisticMove: (taskId: string, toStatus: TaskStatus) => TaskStatus | null
+  rollback: (taskId: string, prevStatus: TaskStatus) => void
+  updatePlan: (taskId: string, plan: string | null) => void
+}
+
+export const useSoloStore = create<SoloStore>((set, get) => ({
+  tasks: {},
+
+  initTasks: (tasks) =>
+    set({ tasks: Object.fromEntries(tasks.map(t => [t.id, t])) }),
+
+  optimisticMove: (taskId, toStatus) => {
+    const prev = get().tasks[taskId]?.status ?? null
+    if (!prev) return null
+    set((s) => ({ tasks: { ...s.tasks, [taskId]: { ...s.tasks[taskId], status: toStatus } } }))
+    return prev
+  },
+
+  rollback: (taskId, prevStatus) =>
+    set((s) => ({ tasks: { ...s.tasks, [taskId]: { ...s.tasks[taskId], status: prevStatus } } })),
+
+  updatePlan: (taskId, plan) =>
+    set((s) => ({ tasks: { ...s.tasks, [taskId]: { ...s.tasks[taskId], implementation_plan: plan } } })),
+}))