Add Vitest and unit tests for skipDisplay.ts

[sbrants]

Summary

src/skipDisplay.ts formats wave jump / skip multipliers for the dashboard, chart, and History. It is pure TypeScript with no test runner today.

Task

1. Add Vitest (or a minimal test setup) as a dev dependency.
2. Add unit tests covering normal jumps (1), banner multipliers (×N), and edge cases (null, large numbers).
3. Wire npm test into CI (.github/workflows/ci.yml) once tests exist.

Why

Frontend logic is currently only checked manually. This is a self-contained module and a safe first contribution.

References

• CONTRIBUTING.md
• src/skipDisplay.ts