fix(editor): 删除项目和账户时清理词库 / clean deleted vocabulary scopes

[zeweihan]

新词库使用 scope_key，不在既有按 projectId/userId 清理的表集合中，删除项目或注销账户后会遗留已学习记录。现在两条删除路径先取得与学习操作相同的父行写锁，再精确清理本范围词库，防止并发写入留下孤儿。账户注销仍须官网传导成功后才清理本地数据。

The learned vocabulary uses scope_key and was omitted from existing project/account cleanup. Acquire the same parent-row write lock used by learning, then delete only the matching scope, preventing concurrent learning from leaving orphan records. Account cleanup still starts only after unified-account deletion succeeds.

验证 / Validation: JDK 21 compile and ProjectDeleteCascadeTest/AccountDeletionServiceTest pass. Real H2 test confirms deleting one project preserves another project's and personal entries; account tests verify lock→vocabulary→user deletion order and zero vocabulary deletion when upstream account deletion fails.

Follow-up to #784; dev-board#538. No new dependencies or schema changes.