Add toolbar items to the Formulas ribbon

[code-by-sia]

Summary

• Replaces placeholder content in the Formulas ribbon with toolbar buttons for supported functions.
• Adds Statistics, Mathematical, Trigonometry, Constants, and User Defined Functions groups.
• Wires formula toolbar buttons to populate the formula bar with formula templates such as =SUM( and =SQRT(.
• Adds regression coverage for Formulas tab toolbar rendering and action dispatch.
• Regenerates GitHub Pages build assets.

Verification

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

Notes:

• npm run build still reports the existing direct eval warning in src/lib/core/formula/Evaluator.ts; build succeeds.
• npm run build-storybook reports large Storybook bundle chunks; the static Storybook build succeeds.