Cherry-picked from PR #452 (fix/canvas-test-and-design-fixes) which
was closed without merge during the PR #443 cascade. The fix adds a
mockPost reference so individual tests can reset the POST mock cleanly
instead of queueing multiple resolved/rejected values.
Without this, the "shows an error toast when POST fails" and "keeps
the card visible when POST fails" tests queue two responses from
beforeEach's mockResolvedValue({}) and the second mockRejectedValueOnce()
call, causing non-deterministic test outcomes.
Fixes test failures in ApprovalBanner suite.