Introduce i18n support, locale switching, and CI for desktop UI

[Copilot]

The desktop UI needs multi-language support with easy locale expansion, plus a user-visible locale switch. This adds translation files and wiring to persist and apply locale selections across renderer and main process, and sets up basic CI to run tests on PRs and pushes.

• i18n foundation
  • Added locale JSON dictionaries and a minimal translation lookup with React context/hook.
• Renderer integration
  • Replaced hardcoded UI strings in dashboard/chat/overlay with t() lookups.
  • Added a locale selector in the dashboard that persists via IPC and loads initial locale.
• Main/IPC plumbing
  • Persist locale in the main process and expose getLocale/setLocale through preload and typed renderer API.
• CI
  • Added a GitHub Actions workflow to run existing tests on PR/push with pinned Bun and minimal permissions.

Example usage:

const { t } = useI18n();
<Typography variant="h6">{t("dashboard.title")}</Typography>

Original prompt

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This section details on the original issue you should resolve

<issue_title>desktop: 多言語化 (i18n) の導入</issue_title>
<issue_description>概要: desktopアプリのUIを多言語化し、複数ロケールに対応します。

目的:

 - 日本語/英語などの翻訳対応
 - 将来的な追加言語対応を容易にする

提案タスク:

 - UI文字列の抽出（desktop/renderer と main 側）
 - 翻訳ファイルの追加（例: locales/en.json, locales/ja.json）
 - ロケール切替UIの実装（設定画面または起動時選択）
 - ビルド/ランタイムでのロケール読み込み実装
 - テストと CI チェック（i18n文字列の漏れ検出など）</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes desktop: 多言語化 (i18n) の導入 #46

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[Berry7028]

Ciも作っておいて

[Berry7028]

@copilot CIも作っておいて

[Copilot]

Ciも作っておいて

Added a CI workflow that runs tests on PR/push with pinned Bun and minimal permissions in commit a1633b0.

@copilot CIも作っておいて

CI workflow is now in place and runs tests on PR/push with pinned Bun and minimal permissions in commit a1633b0.