Fix: prefer Today for header when PickerMonth not explicitly set

Author: w3ori

src/CodeBeam.MudBlazor.Extensions/Components/DateTimePicker/MudBaseDatePickerX.cs

@@ -19,6 +19,7 @@ public abstract partial class MudBaseDatePickerX<T> : MudPicker<T>
     protected readonly string _componentId = Identifier.Create();
 
     internal DateTime? _picker_month;
+    internal bool _picker_month_set;
 
     /// <summary>
     /// Is set to true to scroll to the actual year after the next render
@@ -122,6 +123,7 @@ public DateTime? PickerMonth
             if (value == _picker_month)
                 return;
             _picker_month = value;
+            _picker_month_set = true;
             InvokeAsync(StateHasChanged);
             PickerMonthChanged.InvokeAsync(value);
         }

src/CodeBeam.MudBlazor.Extensions/Components/DateTimePicker/MudDateTimePicker.razor.cs

@@ -466,11 +466,11 @@ private DateTime GetPickerHeaderDate(DateTime? date)
         if (date is not null && (!IsDefaultValue(_value) || date != boundDate))
             return date.Value;
 
-        // Prefer MinDate, then PickerMonth, then local current date.
+        // Prefer MinDate, then PickerMonth (only if explicitly set), then local current date.
         if (MinDate.HasValue)
             return MinDate.Value;
 
-        if (PickerMonth.HasValue)
+        if (PickerMonth.HasValue && _picker_month_set)
             return PickerMonth.Value;
 
         return TimeProvider.GetLocalNow().Date;