feat: Implement UTC conversion for order date and time serialization in order use cases

apps/mobile/packages/core/lib/src/utils/time_utils.dart

@@ -68,3 +68,45 @@ String formatTime(String timeStr) {
     }
   }
 }
+
+/// Converts a local date + local HH:MM time string to a UTC HH:MM string.
+///
+/// Combines [localDate] with the hours and minutes from [localTime] (e.g.
+/// "09:00") to create a full local [DateTime], converts it to UTC, then
+/// extracts the HH:MM portion.
+///
+/// Example: March 19, "21:00" in UTC-5 → "02:00" (next day UTC).
+String toUtcTimeHHmm(DateTime localDate, String localTime) {
+  final List<String> parts = localTime.split(':');
+  final DateTime localDt = DateTime(
+    localDate.year,
+    localDate.month,
+    localDate.day,
+    int.parse(parts[0]),
+    int.parse(parts[1]),
+  );
+  final DateTime utcDt = localDt.toUtc();
+  return '${utcDt.hour.toString().padLeft(2, '0')}:'
+      '${utcDt.minute.toString().padLeft(2, '0')}';
+}
+
+/// Converts a local date + local HH:MM time string to a UTC YYYY-MM-DD string.
+///
+/// This accounts for date-boundary crossings: a shift at 11 PM on March 19
+/// in UTC-5 is actually March 20 in UTC.
+///
+/// Example: March 19, "23:00" in UTC-5 → "2026-03-20".
+String toUtcDateIso(DateTime localDate, String localTime) {
+  final List<String> parts = localTime.split(':');
+  final DateTime localDt = DateTime(
+    localDate.year,
+    localDate.month,
+    localDate.day,
+    int.parse(parts[0]),
+    int.parse(parts[1]),
+  );
+  final DateTime utcDt = localDt.toUtc();
+  return '${utcDt.year.toString().padLeft(4, '0')}-'
+      '${utcDt.month.toString().padLeft(2, '0')}-'
+      '${utcDt.day.toString().padLeft(2, '0')}';
+}