[Epic]: Establish unit testing and initial test coverage

### User Story

As a development team, we want a supported unit-testing approach and initial test suite, so that changes can be validated reliably and regressions caught early.

### Description

Select and configure a unit-test runner, potentially Vitest; establish conventions; and add high-value initial coverage.

Out of scope: end-to-end testing, browser automation, and Playwright.

### Success Metrics

- Runner selected and rationale documented.
- Tests run through the existing workflow and CI where applicable.
- Conventions and guidance documented.
- Initial high-value unit tests added.

### Additional Information

Child Tasks cover audit, runner selection, configuration, conventions, initial coverage, and workflow validation.

Start by reviewing the child tasks for the audit, runner selection, configuration, conventions, initial coverage, and workflow validation; no files or tests are named in the issue. Done means a runner and rationale are documented, tests run through the existing workflow and applicable CI, conventions are documented, and high-value unit tests are added without introducing end-to-end or browser automation.

Written by the indexing model from the issue text.