Localization / i18n scaffolding

[GyaneshSamanta]

Problem

All UI strings are hardcoded English. Non-English-speaking users in India, Brazil, China etc. would benefit from translation.

Proposed approach

• Introduce a tiny `t()` function reading from `src/locales/.json`.
• Replace hardcoded strings in `index.html` and `renderer.js`.
• Settings dropdown for language (default: follow system).
• Start with English + 1–2 community-contributed languages.

Files to touch

• New: `src/locales/en.json`, helper module
• `src/index.html`, `src/renderer.js` — string replacements
• `src/settings.js` — `language` key

Acceptance criteria

• Switching language in settings updates all visible strings without restart.
• Easy to add a new language file via PR.