Add Jest tests and Storybook coverage

[code-by-sia]

Summary

• Replaces the Vitest test runner wiring with Jest, ts-jest, jsdom, and Jest DOM setup.
• Adds focused RTL coverage for shared UI primitives, formula bar editing/abort behavior, and ribbon control command emission.
• Adds Storybook stories for Field, FormulaBar, representative ribbon control groups, and an additional ribbon menu state.
• Adds titles to font style icon buttons so they are accessible and directly testable.
• Rebuilds the GitHub Pages docs bundle.

Validation

• npm test
• npm run typecheck
• npm run build
• npm run build-storybook

Note: npm run build still reports the existing direct eval warning in src/lib/core/formula/Evaluator.ts; it does not block the build. Storybook build also reports the existing large chunk warning for Storybook output.