Expand i18n coverage with new locale files.
Context: i18n infrastructure exists (src/i18n/ with index.ts and locales dir, 120+ strings).
Requirements:
- Extract remaining 500-800 hardcoded UI strings
- Add German, French, Spanish, Dutch locale files
- Language switcher in user preferences
- All new features must use t() for strings
Expand i18n coverage with new locale files.
Context: i18n infrastructure exists (src/i18n/ with index.ts and locales dir, 120+ strings).
Requirements: