Add ability to save and restore canvas state snapshots, allowing users
to preserve their canvas layout at any point and restore it later.
This is useful when the canvas freezes or resets unexpectedly.

Backend:
- Add get_keys_by_prefix and delete_by_key to client_state persistence
- Add corresponding API endpoints

Frontend:
- Add canvasSnapshotRestored reducer to canvasSlice
- Add useCanvasSnapshots hook for snapshot CRUD operations
- Add CanvasToolbarSnapshotMenuButton with save/restore UI
- Add i18n keys for snapshot feature
- Regenerate API schema types

Tests:
- Add tests for new client_state endpoints (prefix search, key deletion)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>