feat(i18n): add Arabic language support

[ahmed5bdelaal]

Summary

This PR adds Arabic (ar) language support.

Changes

• Added Arabic locale.
• Registered Arabic as a supported language.
• Translated all UI strings.
• No breaking changes.

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[chatgpt-codex-connector]

Add the missing jsdom dependency for UI tests

In a clean checkout, the new @page-agent/ui test script is picked up by root npm test/CI, and this config asks Vitest to start the jsdom environment; Vitest documents that this environment uses the jsdom package (docs), but the repo only declares happy-dom and vitest, not jsdom, so the UI test workspace will fail before running any tests. Add jsdom to the repo devDependencies/lockfile or use the already-installed happy-dom/default node environment instead.

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