fix: persist whiteboard history (per-stage) and whiteboard content across refresh

[YizukiAme]

Summary

Fix whiteboard data loss on page refresh by:

1. Persisting whiteboard history per-stage — Refactored whiteboard-history.ts to store snapshots grouped by stageId (via snapshotsByStage: Record<string, WhiteboardSnapshot[]>) and added Zustand persist middleware with localStorage backend.

2. Persisting current whiteboard content to IndexedDB — Added whiteboard?: Whiteboard[] field to StageRecord in database.ts and included data.stage.whiteboard in saveStageData() in stage-storage.ts. Previously, stage.whiteboard was silently dropped during serialization because StageRecord didn't include this field.

Changes

Whiteboard History Persistence (per-stage)

File	Change
lib/store/whiteboard-history.ts	Refactored: snapshots → snapshotsByStage, all actions now accept stageId. Added persist middleware (localStorage key: openmaic-whiteboard-history).
components/whiteboard/index.tsx	Pass stageId to pushSnapshot() and getSnapshots().
components/whiteboard/whiteboard-history.tsx	Pass stageId to getSnapshots() and getSnapshot().
components/whiteboard/whiteboard-canvas.tsx	Pass stageId to auto-snapshot pushSnapshot() call.
lib/action/engine.ts	Pass stageId to pushSnapshot() in executeWbClear().

Whiteboard Content Persistence (IndexedDB)

File	Change
lib/utils/database.ts	Added whiteboard?: Whiteboard[] to StageRecord interface.
lib/utils/stage-storage.ts	Include data.stage.whiteboard when saving to db.stages.

Testing

• pnpm tsc --noEmit passes
• History snapshots persist across page refresh
• History is isolated per-stage (switching stages shows correct history)
• Whiteboard content persists across page refresh
• IndexedDB stages table contains whiteboard field after save
• localStorage key openmaic-whiteboard-history contains per-stage snapshots