Add English localization (i18n)

[lvchenjia]

Description

The entire UI is currently in Chinese. We need to add English localization using the standard String(localized:) / LocalizedStringKey approach.

Tasks

• Enable String(localized:) in all Text views in ContentView.swift and BrowserProxyApp.swift
• Create en.lproj/Localizable.strings with English translations
• Create zh-Hans.lproj/Localizable.strings with the existing Chinese text
• Verify the app automatically picks the right language based on system locale

Resources

• All UI text is in Sources/BrowserProxy/ContentView.swift (~340 lines)
• Reference: Apple's documentation on LocalizedStringKey

Testing

• Change system language to English, launch the app, verify all text is in English
• Change back to Chinese, verify Chinese text is restored