Close testing gaps: error paths, undo system, edge cases

[svrnm]

Summary

Audit test coverage and add missing tests for under-tested areas of the codebase.

Areas to investigate

• Command error paths — Most command tests cover the happy path. Add tests for malformed inputs, missing DOM elements, and invalid selectors.
• Undo system — UndoElement and the undo tracking in Monkey.js lack dedicated tests. Verify that disabling a config fully reverts all changes.
• Configuration parsing edge cases — Test malformed INI, circular imports, duplicate options, and variable resolution failures.
• E2E expansion — Current E2E tests cover basic flows. Add scenarios for multiple active configs, config toggling, and popup interactions.

Approach

1. Run coverage analysis to identify the biggest gaps
2. Prioritize tests for code paths that affect correctness (undo, state mutation)
3. Add E2E tests for user-facing workflows not yet covered