fix(#7): use UTC for DateField initial seed to match pick conversion

DateField seeded the picker with ZoneId.systemDefault() but converted
the selectedDateMillis back with ZoneId.of("UTC"). For any non-UTC
user, the field and the picker displayed different days, and
confirming without re-picking silently shifted the export range by a
day. Use UTC on both sides (the DatePickerState contract is that
selectedDateMillis is UTC midnight of the picked day).

app/src/main/java/dev/achmad/ledgerr/ui/components/ExportAction.kt

@@ -185,7 +185,7 @@ private fun DateField(
     )
 
     if (showPicker) {
-        val initialMillis = date.atStartOfDay(ZoneId.systemDefault())
+        val initialMillis = date.atStartOfDay(ZoneId.of("UTC"))
             .toInstant()
             .toEpochMilli()
         val datePickerState = rememberDatePickerState(initialSelectedDateMillis = initialMillis)