Add configurable responsive breakpoints

[JLG-WOCFR-DEV]

Summary

• add a reusable helper that retrieves configurable desktop/tablet/mobile breakpoints with sanitization and filtering
• expose the breakpoints to the utilities script and update the responsive CSS/preview logic to consume them dynamically
• update REST handling and tests, including a new Playwright scenario, to cover custom breakpoint usage

Testing

• composer test (fails: WordPress test database not available in container)
• npx playwright test tests/ui/responsive-preview.spec.js --project=chromium (fails: Docker/wp-env unavailable in container)

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https://chatgpt.com/codex/tasks/task_e_68dd42522be4832e8a6c51cf2e322635