Remove broken i18n system

[tmshv]

Problems

1. i18n block in next.config.js is dead code — next.config.js:57-61
   This is a Pages Router feature. App Router ignores it and logs a build warning. Remove the entire i18n block.

2. translationKey on menu items is never used — src/constants.ts, src/app/layout.tsx
   Menu items define translationKey but MenuItem component never reads it. Remove the property from all menu item objects and the type.

3. LangContext created but never provided or consumed — src/context/lang.ts
   Delete this file. No component uses it.