Add trilingual localization support

[P013onEr]

Summary

• Add shared localization support for Simplified Chinese, English, and Japanese.
• Add automatic language detection plus manual language selection in Settings.
• Share the selected language through App Group so the main app and Finder Sync extension stay consistent.
• Localize settings, menu bar actions, Finder context menu sections, updater UI, permission prompts, alerts, and default file names.

Verification

• Validated all AppText.strings files with plutil.
• Checked there are no remaining hard-coded user-facing Chinese SwiftUI/NSAlert strings.
• Built successfully with: xcodebuild -project RClick.xcodeproj -scheme RClick -configuration Debug -derivedDataPath work/DerivedData-Verify CODE_SIGNING_ALLOWED=NO CODE_SIGNING_REQUIRED=NO build

[wflixu]

Thank you so much for your contribution! Adding trilingual localization support is exactly what RClick needs 🙌

One suggestion: since the project has already dropped compatibility for macOS versions below 15, I'd recommend using the .xcstrings format instead of the traditional .strings files. It's Apple's recommended format since Xcode 15 — JSON-based, easier to manage, and removes the need to maintain both AppText.strings and Localizable.xcstrings separately.

Would you like to update this PR to use .xcstrings, or I can merge it as-is and migrate myself — whichever works best for you.

[P013onEr]

Updated the PR to use Localizable.xcstrings for both the main app and FinderSync extension, and removed the previous AppText.strings files. I also verified the string catalogs with xcstringstool and confirmed the project builds locally.