Phase 2: Add config support for week_start_day and default_due_time

- Add WeekStartDay and DefaultDueTime fields to Config struct
- Set defaults: week_start_day=monday, default_due_time=empty
- Add GetWeekStart() and GetDefaultDueTime() helper methods
- Update ParseDate() to use configured week start day
- All tests passing

opal-task/internal/engine/dateparse.go

@@ -263,8 +263,14 @@ func (p *DateParser) nextWeekday(target time.Weekday) time.Time {
 	return time.Date(next.Year(), next.Month(), next.Day(), 0, 0, 0, 0, next.Location())
 }
 
-// ParseDate is the public API that uses default settings
+// ParseDate is the public API that uses config settings
 func ParseDate(s string) (time.Time, error) {
-	parser := NewDefaultDateParser()
+	cfg, err := GetConfig()
+	weekStart := time.Monday // default
+	if err == nil {
+		weekStart = cfg.GetWeekStart()
+	}
+
+	parser := NewDateParser(timeNow(), weekStart)
 	return parser.ParseDate(s)
 }