feat: add system-aware dark mode and complete localization

[iamkunal9]

Summary

• Add persisted System / Light / Dark theme selection.
• Follow the operating system theme while System mode is selected.
• Apply semantic light and dark color tokens across the UI.
• Add translated theme labels for all supported locales.
• Localize runtime status and error messages returned by the native backend.
• Prevent raw Chinese backend errors from appearing in English, Japanese, or Traditional Chinese UI.
• Use locale-aware translation fallbacks: Traditional Chinese falls back to Traditional Chinese, while Japanese and English fall back to English.
• Add regression coverage for runtime message localization.

Verification

• bun test scripts/runtimeText.test.ts
• bun run check
• bun run build
• Manual browser smoke test for English, Traditional Chinese, Japanese, and Simplified Chinese switching.
• Confirmed English mode renders no visible Chinese text on the initial UI.