Add Russian localization

[Dobrunia]

Summary:

Add full Russian (ru.json) translation to the app and connect it to the existing language switcher.

Tasks:

• Copy en.json → ru.json and translate all strings.
• Register Russian locale in i18n/index.ts.
• Make the switcher toggle between EN/RU and remember the choice (e.g. localStorage).
• Ensure fallback to English for missing keys.

Result:

Users can switch interface language between English and Russian.