Add I18n support for all UI strings

## Description
All strings are hardcoded in English (flash messages, labels, UI text). Should use `I18n.t` for internationalization.

## Acceptance Criteria
- [ ] All flash messages use I18n keys
- [ ] View text (headings, labels, buttons) uses I18n keys
- [ ] Default English locale file provided at `config/locales/en.yml`
- [ ] Host apps can override translations