feat: add Simplified Chinese (zh-CN) internationalization

[superpickyapp]

Summary

This PR adds full Simplified Chinese (简体中文) support to RapidRAW using react-i18next, while keeping English as the default language. The language can be switched instantly from the Settings panel without restarting the app, and the preference is persisted in AppSettings.

What's included

• i18n framework: Added i18next + react-i18next, configured in src/i18n.ts with en as default and zh-CN as the second locale
• Translation files: src/locales/en/translation.json and src/locales/zh-CN/translation.json — covering all namespaces: app, metadata, library, editor, adjustments, crop, export, presets, ai, settings, tooltips, modals, masks
• Language persistence: Added language: Option<String> field to Rust AppSettings struct; language is loaded on startup and saved on change
• Settings UI: Language dropdown in Settings panel (English / 简体中文), switches instantly via i18n.changeLanguage()
• Full coverage: All hardcoded English strings replaced with t() calls across 50+ components — including panels, modals, context menus, adjustment controls, mask type names, waveform modes, color labels, and more

Architecture notes

• Module-level constant arrays that contain display labels (e.g. mask types, backend options) use a nameKey / labelKey pattern — the key is stored in the array and t(key) is called at render time, so language switches are reactive
• Native macOS menu bar (File/Edit/Window) follows the OS system language — this is intentional and consistent with how other macOS apps behave
• The name field on MaskType is preserved for backwards compatibility; nameKey is the new field used for translated display

How to test

1. Build and run the app
2. Open Settings → scroll to Language → select 简体中文
3. The entire UI should switch to Chinese immediately
4. Restart the app — language preference is restored from saved settings
5. Switch back to English — everything returns to English

I hope this can be useful to Chinese-speaking users of RapidRAW. Happy to address any feedback or make adjustments to fit the project's conventions.

🤖 Generated with Claude Code