Add website internationalization

[Jamesllllllllll]

Summary

Add website internationalization with persisted locale preferences, SSR locale bootstrapping, localized route metadata, and translated public/dashboard/admin surfaces.

Refs #59.

What Changed

• add i18next + react-i18next + i18next-icu website i18n infrastructure
• add explicit locale persistence with cookie, local storage, and user preference storage
• add SSR locale bootstrapping to remove the flash of English on localized pages
• add a shared language picker in the app header
• translate the website shell, home, search, public playlist, dashboard, settings, and admin pages for en, es, fr, and pt-BR
• add i18n formatting helpers, metadata helpers, missing-key dev warnings, locale tests, and a manual coverage check script
• keep bot replies and the Twitch panel out of scope for this PR

How To Test

1. Run npm run dev.
2. Open the home page, search page, a public playlist page, and dashboard/admin/settings pages.
3. Change the language from the header picker and confirm the selection persists across navigation and refresh.
4. Hard refresh a localized page and confirm it renders in the selected language without flashing English first.
5. Navigate between internal pages like home, search, playlist, and settings and confirm the signed-in header state stays warm.
6. Run npm run check:prepush.

Screenshots Or UI Notes

Adds a header language picker and localized copy across the website surfaces listed above. This PR intentionally does not localize bot replies or the Twitch panel.

Checklist

• I tested the change locally
• I updated docs or examples if needed
• I called out any follow-up work or known limitations

[github-actions]

Preview deployment updated.

App: https://request-bot-pr-65.jameskeezer.workers.dev
Frontend worker: request-bot-pr-65
Backend worker: request-bot-backend-pr-65