Tighten exception handling flagged by Ruff

[ZFordDev]

Closes #179

This PR narrows optional UI imports to ImportError, logs resilient reminder failures, restores failed undo actions instead of silently discarding them, adds undo coverage, and documents the exception-boundary policy.

This PR is stacked on #182 to keep its review isolated.

Validation:

• BLE001, S110, and stale suppression checks pass across src and tests.
• Ruff formatting passes.
• 20 relevant tests pass locally; the real Windows message-box test is excluded because it opens a modal OS dialog.
• Only the three PyQt model findings assigned to Resolve Ruff safety findings in the PyQt task model #180 remain in the full Ruff result.