feat(i18n): implement dynamic locale loading, local JSON imports, and customized window title

[Thanhdv2007]

Description

This Pull Request implements a comprehensive, dynamic internationalization (i18n) management system for Codex++ Manager. Instead of compile-time static translations, this system enables runtime loading, hot-swapping, and user-importing of translation resource files (.json) directly from the disk.

Key Changes

1. Backend (Rust / Tauri)

• Dynamic Locale Loading (load_custom_locales command): Scans the locales/ directory located adjacent to the executable binary. Any valid .json locale files found are loaded into the frontend dynamically at startup.
• Locale Import Dialog (import_custom_locale command): Exposes a secure Tauri API triggering a native OS file dialog. Users can select any local translation JSON file, which is then parsed, validated, and copied into the application's locales/ directory for persistence.
• Tauri Command Registration: Properly registered commands in src-tauri/src/lib.rs.

2. Frontend (React / i18next)

• Custom Dropdown Selector:
  • Replaced the generic HTML select box with a styled, responsive React dropdown component.
  • Optimized width (fixed at ~85px) and simplified display to uppercase locale codes (e.g., ZH, EN, VI).
  • Implemented clicking outside detection to automatically close the dropdown menu.
  • Applied position: "relative" and z-index: 50 on the topbar header to prevent the dropdown from being clipped or covered by the layout view below.
• On-the-fly Bundle Injection: The React app listens for imported locales, registers the bundle dynamically using i18nInstance.addResourceBundle, and switches the active language immediately without requiring a restart.
• Dynamic Window Title:
  • Added a useEffect that listens for language changes and updates the Tauri window title dynamically.
  • Configured default window title in tauri.conf.json as Codex++ Manager (neutral start), updating to "Codex++ 管理工具" for Chinese (ZH) and "Codex++ Manager" for English (EN), Vietnamese (VI) or any other imported language.

3. Preset & Provider Localization

• Refactored static category label maps inside ProviderPresetSelector.tsx to read translation keys dynamically.
• Localized static branding strings for model providers (e.g., Volcengine, Baidu, Xiaomi) in presets.ts.
• Completely translated remaining UI elements to English and Vietnamese.

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Verification & Build Status

• Ran npm run check (TypeScript verification) - PASSED
• Ran npm run vite:build (Vite production bundle build) - PASSED
• Ran cargo check (Rust compilation verification) - PASSED