* Fix date format inconsistency in Task detail screen (#938)
Replace browser-dependent toLocaleDateString() with explicit country-based
date formatting to ensure consistent date formats based on user's timezone.
Problem:
- User with English language + Greek timezone saw MM/DD/YYYY format
- Expected DD/MM/YYYY format based on timezone/country
- Browser's Intl.DateTimeFormat had incomplete locale support for
combined locales like "en-GR"
Solution:
- Add country-to-format mapping in dateUtils.ts (60+ countries)
- New formatDateByCountry() for dates (DD/MM/YYYY, MM/DD/YYYY, YYYY/MM/DD)
- New formatDateTimeByCountry() for datetimes with 24h time
- Update TaskDueDateCard and TaskDeferUntilCard to use new functions
- Uses date-fns for consistent cross-browser formatting
Testing:
- Added 40 comprehensive test cases covering all format types
- Verified with Greece (DD/MM), US (MM/DD), Japan (YYYY/MM/DD)
- All tests passing
Fixes#938
* chore: remove unused import in dateUtils.ts
Date fields in the task edit page used i18n.language (e.g. "en") to
determine date format, giving MM/DD for English even when the user's
timezone indicates a DD/MM region (e.g. Europe/Athens).
Enhance resolveUserLocale to derive the country from the user's
timezone and combine it with the language (e.g. "en" + "GR" = "en-GR")
so date formatting follows regional conventions.
