Add i18n support, fix linger timing, and add topmost toggle

[chan-yuu]

Summary

• add lightweight i18n support with Simplified Chinese and use system language by default
• fix key linger expiration so released keys disappear after the configured duration
• stop grouping normal sequential letters into the same combo and disable text ligatures to avoid merged labels like RU or HI
• add an always-on-top toggle for the overlay and keep it enabled by default

Details

• introduce a small frontend i18n layer and wire the main settings pages to Chinese and English labels
• fix release timestamp handling in the key event store so linger duration applies reliably
• restrict combo grouping to modifier and mouse-related combinations instead of normal letter sequences
• apply runtime window alwaysOnTop updates from settings so the overlay can stay above other apps

Verification

• npm run build
• npm run tauri build