Arabic language support & RTL direction support

## Feature summary
Add Arabic language (العربية) interface translation and full Right-to-Left (RTL) layout support to the GitHub Copilot app.

## What problem are you trying to solve?
Arabic is the 4th most spoken language in the world with over 400 million native speakers, and many Arab developers rely on GitHub Copilot daily. Currently, the app is English-only with a Left-to-Right (LTR) layout. This makes navigation feel unnatural and harder to follow for Arabic-speaking users, especially those less comfortable with English UIs. Additionally, for developers who use Arabic alongside English in their workflow — such as writing Arabic commit messages, comments, or documentation through Copilot — having the interface correctly support RTL handling is essential.

## Proposed solution
- Add Arabic (ar) locale support for the full app interface (menus, settings, session UI, canvas elements, etc.)
- Implement RTL layout support so that the UI correctly mirrors: text alignment, navigation direction, padding/margins, and input elements
- Use a standard i18n framework (e.g., i18next or similar) that makes adding future languages straightforward
- Ensure Copilot's conversation responses and code generation remain unaffected (still work in any language)

## Workflow impact
This benefits the entire Arabic-speaking developer community using GitHub Copilot — from students and independent developers to enterprise teams across the Middle East and North Africa. It also establishes an i18n foundation that makes adding other RTL languages (like Hebrew, Urdu, Persian) much easier in the future.

## Installation context
Desktop app — affects all installation contexts (local mode, cloud sessions)

## Additional context
I'm an Arabic-speaking developer and I'm willing to contribute to implementing this feature if the team is open to community contributions. I can help with:
- Arabic translation/localization
- RTL layout testing and QA
- i18n framework integration
- Testing with real-world Arabic workflows

从桌面应用现有的界面开始，重点关注 issue 中提到的本地模式和云会话。定义 Arabic locale、i18n 方案，以及菜单、设置、会话 UI、画布元素、文本对齐、导航、间距和输入中的 RTL 行为；完成标准是整个界面都已翻译并正确镜像，同时不改变对话响应或代码生成。

由索引模型根据 Issue 内容生成。