Add date/time format selection in Settings #9
Description
Problem
Date and time formats are currently hardcoded (e.g., "HH:mm", "Monday, Mar 3"). Users in different regions may prefer different formats (12h vs 24h, date ordering).
Proposal
Add a setting to choose between:
- 24-hour / 12-hour time format
- Date format options (e.g., "Mar 6" vs "6 Mar" vs "06/03")
- Or: follow system locale defaults
Acceptance criteria
- New preference in Settings screen for date/time format
- All date/time displays respect the chosen format
- Sensible default (system locale or 24h)