Fix vertical alignments

src/Avalonia.Themes.Fluent/Controls/CalendarButton.xaml

@@ -11,7 +11,7 @@
     <Setter Property="MinWidth" Value="40"/>
     <Setter Property="MinHeight" Value="40"/>
     <Setter Property="Margin" Value="1"/>
-    <Setter Property="Padding" Value="0,0,0,4"/>
+    <Setter Property="Padding" Value="0,0,0,0"/>
     <!--These are actually set on the CalendarView in WinUI-->
     <Setter Property="Foreground" Value="{DynamicResource CalendarViewCalendarItemForeground}"/>
     <Setter Property="Background" Value="{DynamicResource CalendarViewCalendarItemRevealBackground}"/>

src/Avalonia.Themes.Fluent/Controls/CalendarDayButton.xaml

@@ -11,7 +11,7 @@
     <Setter Property="MinWidth" Value="40"/>
     <Setter Property="MinHeight" Value="40"/>
     <Setter Property="Margin" Value="1"/>
-    <Setter Property="Padding" Value="0,0,0,4"/>
+    <Setter Property="Padding" Value="0,0,0,0"/>
     <!--These are actually set on the CalendarView in WinUI-->
     <Setter Property="Foreground" Value="{DynamicResource CalendarViewCalendarItemForeground}"/>
     <Setter Property="Background" Value="{DynamicResource CalendarViewCalendarItemRevealBackground}"/>

src/Avalonia.Themes.Fluent/Controls/CalendarItem.xaml

@@ -32,6 +32,7 @@
             <Style Selector="Button.CalendarHeader">
               <Setter Property="HorizontalAlignment" Value="Stretch" />
               <Setter Property="VerticalAlignment" Value="Stretch" />
+              <Setter Property="VerticalContentAlignment" Value="Center" />
               <Setter Property="FontSize" Value="20" />
               <Setter Property="Background" Value="{DynamicResource CalendarViewNavigationButtonBackground}"/>
               <Setter Property="Template">