feat: add breakpoint tokens and hook

[silentgeckoaudit3801]

Closes #471

Summary

• add a breakpoint token set under design-system/tokens/breakpoints.json
• register breakpoint tokens in the token loader/type catalog and expose --breakpoint-* CSS variables
• add an SSR-safe useBreakpoint hook plus focused token/loader/hook tests and docs

Verification

• Static API inspection confirmed the token JSON parses and expected references are present.
• I did not install dependencies or run the project locally in this environment.