Add Simplified Chinese localization

[makerjackie]

Summary

• add a Simplified Chinese (zh-CN) translation table covering the existing settings/onboarding/browser setup/app-blocking copy
• generalize the language picker from a two-language toggle into a multi-language option list
• use locale-aware formatting for zh-CN browser lists, counts, remaining time, and override challenge units

Verification

• npm run vite:build
• cargo check
• node --check src/app.js
• translation key coverage check: zh-CN covers all English translation keys
• built and installed a local aarch64 macOS app at /Applications/ReDD Blocker.app using ad-hoc signing

Note: the default universal macOS build needs the x86_64-apple-darwin Rust target; this local machine has Homebrew Rust without that target, so local installation was verified with BUILD_MAC_TARGET=aarch64-apple-darwin APPLE_SIGNING_IDENTITY_OVERRIDE=-.