- BulkUpdate handler: add structured details to 409 response
- BulkUpdateAccounts: simplify to global pre-check before any DB write;
remove per-account snapshot tracking which is no longer needed
- MixedChannelError.Error(): restore English message for API compatibility
- BulkEditAccountModal: use t() with details for both pre-check and 409
fallback paths instead of displaying raw backend strings
- Update test to verify pre-check blocks on existing group conflicts
补充批量调度返回 success_ids/failed_ids 并增加合约/单测
前端加入降级处理与部分失败提示，表格行使用稳定 key
测试: make test-frontend
测试: go test ./internal/service -run BulkUpdateAccounts -tags=unit
测试: go test ./internal/server -run APIContracts -tags=unit
