Add locale switcher to DatePicker demo

[jaymantri]

Summary

• replace the hardcoded French DatePicker demo with a locale-switcher example
• add a localized demo that supports multiple locales and week start settings
• keep the change scoped to src/app/page.tsx

Testing

• attempted targeted lint/build validation
• validation is currently blocked in this environment because dependencies are not installed and CENTRAL_LICENSE_KEY is not set for npm ci

  

[jaymantri]

Closing as superseded by the canonical Origin DatePicker work in lightsparkdev/webdev#31617 and its locale-hardening follow-up lightsparkdev/webdev#32241. This branch is still clean against standalone origin/main, so this is not a conflict- or age-based closure. Its only unique idea is the multi-locale manual-QA selector; if that remains useful, we should reimplement it in webdev Storybook rather than cherry-pick this one-file demo commit.