Add support for cancelling transaction groups

[jbair06]

Description

The application currently only supports cancelling a single transaction at a time. To improve usability and align with existing “sign all” behavior, the system must support cancelling an entire transaction group which is triggered by the “cancel all” action.

Problem

• The UI and API endpoint only allow cancelling a single transaction.
• There is no way for users to cancel all transactions in a group in a single api call.
• Naively looping over single‑cancel calls would be inefficient and harder to keep consistent with existing “sign all” behavior.

Proposed Solution

• Extend the API and backend logic to support cancelling a transaction group.
• Implement “cancel group” using grouped SQL operations, similar to the existing “sign all” logic, to minimize per‑transaction overhead and keep behavior consistent.
• Update the frontend to call the new “cancel all” endpoint
• Update current "cancel all" tests to use new endpoint and logic

Acceptance Criteria

• API supports cancelling a transaction group operation in addition to single cancel.
• SQL is batched/grouped for cancel group, following the same pattern as “sign all”.
• New and existing tests pass, covering single cancel, group cancel, and cancel all flows.