Add amount token formatter component

[220nightmore-spec]

Summary

• add a reusable AmountFormatter design-system component
• support locale-aware number formatting, token symbols, raw token scaling, compact notation, signed values, empty-value fallbacks, and tone classes
• wire CampaignCard reward display through the formatter
• add Vitest coverage and Storybook stories for the component

Validation

• node node_modules/vitest/vitest.mjs run src/components/AmountFormatter.test.jsx --config vitest.no-setup.config.mjs (temporary no-setup config; 6 tests passed)
• node node_modules/eslint/bin/eslint.js src/components/AmountFormatter.jsx src/components/AmountFormatter.test.jsx src/components/CampaignCard.jsx src/stories/AmountFormatter.stories.jsx src/lib/amountFormatter.js
• git diff --check

Notes

The repository's default frontend Vitest setup currently imports @testing-library/jest-dom, but that package is not declared in frontend/package.json. The main Vite/Storybook builds are also blocked by existing dependency/build configuration issues (workbox-window / PWA plugin resolution). This PR keeps the component scope focused and does not change unrelated dependencies.

Closes #977