feat(i18n): add Arabic (ar-SA) translation

[bigdizaya]

Summary

• Add complete Arabic (ar-SA) translation file (backend/chainlit/translations/ar-SA.json)
• All keys match the en-US.json reference structure
• Arabic is one of the most spoken languages worldwide (400M+ native speakers) and is currently missing from Chainlit's supported translations

Details

This PR adds a single translation file following the exact same pattern as other translation PRs (#1322 he-IL, #2308 fr-FR, #2458 es, #2517 de-DE, #2534 ko, #2646 it).

The translation covers all UI sections:

• Common actions (cancel, confirm, submit, etc.)
• Authentication (login form, errors, OAuth)
• Chat interface (input, favorites, commands, speech, file upload, messages, feedback)
• Thread history (sidebar, timeframes, thread actions: share, delete, rename)
• Navigation (header, new chat, user menu)
• API keys, alerts, components

Note on RTL

This PR focuses on translation only. RTL (right-to-left) layout support would be a separate, more involved change. Adding the translation file is the first step — it enables Arabic-speaking users to see UI text in their language, even if the layout direction remains LTR for now.

Closes #2781

Test plan

• Verify the JSON file is valid and matches the en-US.json structure
• Set browser language to ar-SA and confirm translations load via /project/translations?language=ar-SA

🤖 Generated with Claude Code

---

Summary by cubic

Add full Arabic (ar-SA) translation for the Chainlit UI with keys mirroring en-US.json. This fulfills #2781 by enabling Arabic text; RTL layout will be handled separately.

• New Features
  • Added backend/chainlit/translations/ar-SA.json with full parity to en-US.json.
  • Covers common actions, auth, chat, thread history, navigation, API keys, alerts, and components.

^{Written for commit 58ed9df. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 1 file