feat(saas): Phase 4 端到端完善 — 设备注册、离线支持、配置迁移、集成测试

- 后端: devices 表 + register/heartbeat/list 端点 (UPSERT 语义)
- 桌面端: 设备 ID 持久化 + 5 分钟心跳 + 离线状态指示
- saas-client: 重试逻辑 (2 次指数退避) + isServerReachable 跟踪
- ConfigMigrationWizard: 3 步向导 (方向选择→冲突解决→结果)
- SaaSSettings: 修改密码折叠面板 + 迁移向导入口
- 集成测试: 21 个测试全部通过 (含设备注册/UPSERT/心跳、密码修改、E2E 生命周期)
- 修复 ConfigMigrationWizard merge 分支变量遮蔽 bug

crates/zclaw-saas/src/account/mod.rs

@@ -17,4 +17,7 @@ pub fn routes() -> axum::Router<crate::state::AppState> {
         .route("/api/v1/tokens/{id}", delete(handlers::revoke_token))
         .route("/api/v1/logs/operations", get(handlers::list_operation_logs))
         .route("/api/v1/stats/dashboard", get(handlers::dashboard_stats))
+        .route("/api/v1/devices", get(handlers::list_devices))
+        .route("/api/v1/devices/register", post(handlers::register_device))
+        .route("/api/v1/devices/heartbeat", post(handlers::device_heartbeat))
 }