feat(i18n): add Arabic (ar-SA) language support

[YizukiAme]

Summary

Add Arabic (ar-SA) language support to the i18n system, addressing #427.

Changes

New file

• lib/i18n/locales/ar-SA.json — Complete Arabic (Saudi Arabia) translation with all 803 i18n keys

Modified file

• lib/i18n/locales.ts — Register ar-SA locale entry (العربية / AR)

Translation Quality

• Base translation generated with LLM, then cross-validated by 3 independent LLMs with domain expertise in Arabic localization
• 15 targeted corrections applied from review feedback, including:
  • Terminology consistency (Provider → المزوّد uniformly)
  • RTL punctuation fixes (exclamation mark positioning)
  • Translation error fixes (عادلاً → عاليًا in voice description)
  • streaming connotation fix (البث المباشر → التدفق)
  • Expanded TTS/ASR abbreviations in user-facing labels
  • Arabic comma (،) instead of English comma (,)
• All {{interpolation}} variables preserved and verified
• Brand names and provider names kept in English per convention
• Language names kept in native script

Known Limitations

1. RTL layout: This PR only adds the translation file. The UI layout remains LTR. Full RTL support (flex direction, text alignment, margins, icon positioning) is a systematic UI change that should be tracked as a separate issue.
2. Arabic plural forms: Arabic has singular/dual/few/many/other plural rules. The current i18n keys use a single form. Adding proper plural support (_one/_two/_few/_many/_other suffixes) requires both translation additions and code changes to pass count parameters — tracked as a future enhancement.
3. Voice descriptions: Some Chinese-cultural voice descriptions (e.g., cross-talk, regional dialects) are translated literally. Native speaker review for cultural adaptation is welcome.

Verification

• All 803 keys match en-US.json exactly (automated verification)
• TypeScript compilation passes with zero errors
• JSON structure valid and well-formed

Closes #427

Translated and audited by Claude Opus 4.6, GPT 5.4 and Gemini 3.1 Pro.
Manually researched and audited.

[wyuc]

Rebase onto current main. A few PRs landed since this was opened:

Missing 12 keys (added by #418 — classroom ZIP export/import):

export.classroomZip
export.classroomZipDesc
import.classroom
import.error.invalidManifest
import.error.invalidZip
import.error.missingData
import.error.storageFull
import.parsing
import.success
import.validating
import.writingCourse
import.writingMedia

Remove 2 stale keys (deleted in #411/#412):

pbl.chat.welcomeMessage
toolbar.languageHint

[YizukiAme]

working on it!

[YizukiAme]

Rebased onto current main and addressed all feedback~

• Added 12 missing keys from feat: classroom export and import (ZIP) (#408) #418
• Removed 2 stale keys
• 813/813 keys match