feat: add arabic to the shared contact page

[leviFrosty]

Summary

WitnessWork is adding Arabic as its first right-to-left language (app PR: leviFrosty/witness-work, the rtl-arabic branch). This is the backend part: the shared-contact page (/c and legacy /c/<payload>) now has Arabic text.

• contactLinkLocales.ts: adds an ar entry. Like the app, there is one Arabic for every country, so the key has no region. matchLocale now also matches a key that is just the language, so ar, ar-SA and ar-EG (from ?lang= or Accept-Language) resolve to ar.
• contactLink.ts: right-to-left locales render <html lang="ar" dir="rtl">. Other locales are unchanged. og:locale for Arabic is ar_AR, the code Open Graph uses for Arabic.
• Tests: Arabic renders on both routes with dir="rtl", plus Accept-Language negotiation for ar, ar-EG and xx, AR-sa;q=0.6.

The app never sends lang on contact links, so the recipient's browser language decides. Without this change, Arabic browsers get the English page. That works, but it's a gap for an Arabic launch.

Verification

• pnpm vitest run: 56 files, 725 tests passed. pnpm typecheck is clean.
• verify-ww-api: node scripts/verify/dev.mjs up --persist-to /tmp/ww-verify-arabic, then doctor was all ok. Over HTTP:
  • GET /c with Accept-Language: ar-SA,ar;q=0.9 returned 200 with Content-Language: ar, <html lang="ar" dir="rtl">, <title>افتح جهة الاتصال هذه في WitnessWork</title> and og:locale ar_AR.
  • GET /c with Accept-Language: en-US still returned <html lang="en-us"> (no dir).
• Not verified: how the page looks in a browser. No headless browser was available on the machine.

Land this on main before the app's Arabic PR ships, per the app's release order (scripts/release-api.sh).