Auto-detect user timezone for date/time inputs#1560

Currently, the date/time pickers across the platform use inconsistent timezone handling. The Maintenance Window scheduler uses UTC, while the Announcement form claims to use GMT+3 but actually uses the user’s local timezone. Instead of requiring users to mentally convert between timezones, the app should auto-detect the user’s timezone and display all dates/times in their local time, with the timezone clearly labeled (e.g. “March 11, 2026, 01:00 AM UTC-3”). This would prevent scheduling mistakes and provide a consistent experience across all features.