Add a breakpoints design token set and a useBreakpoint hook

[1nonlypiece]

📌 Description

Responsive behaviour is documented in breakpoints.md but breakpoint values
are not tokenized, and there is no runtime hook to branch layout on the current
breakpoint. This issue adds a breakpoints.json token file and a
useBreakpoint hook consuming it.

🎯 Requirements and Context

• New design-system/tokens/breakpoints.json (sm/md/lg/xl) matching the
  documented values; register in getAllTokens.
• A useBreakpoint() hook in the app returning the active breakpoint via
  matchMedia, SSR-safe and cleanup-correct.

🛠️ Suggested Execution

1. Fork the repo and create a branch

git checkout -b feature/breakpoint-tokens

2. Implement changes

• Add design-system/tokens/breakpoints.json; update token-loader.
• New src/utils/useBreakpoint.ts.
• Tests: extend design-system/src/__tests__/token-loader.test.ts; add
  src/utils/__tests__/useBreakpoint.test.tsx.
• Docs: extend design-system/documentation/breakpoints.md.

3. Test and commit

• Run the design-system Jest suite and npm test.
• Cover edge cases: matchMedia change events, listener cleanup, boundary widths,
  default when matchMedia missing.

Example commit message

feat: breakpoint tokens and useBreakpoint hook with tests

✅ Guidelines

• Minimum 95% test coverage on the new/changed lines.
• SSR-safe; clean listener teardown.
• Timeframe: 96 hours.

🏷️ Labels

type-feature · area-frontend · MAYBE REWARDED · GRANTFOX OSS · OFFICIAL CAMPAIGN

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[silentgeckoaudit3801]

I'd like to take this issue if it is still available. I will wait for assignment/maintainer confirmation before opening a PR, matching the campaign's assigned-work review flow. Planned scope: DTCG breakpoint tokens aligned with the existing breakpoint docs, registration in getAllTokens, an SSR-safe useBreakpoint hook using matchMedia, focused hook/listener cleanup tests, and documentation updates.