Fix: FAB white background with dark icon in dark mode

Add FabBackgroundColor / FabTextColor theme keys so the FAB is always
visible: dark background + white icon in light mode, white background +
dark icon in dark mode.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Xamarin.Neo4j/Xamarin.Neo4j/Xamarin.Neo4j/Pages/ConnectionsPage.xaml

@@ -142,8 +142,8 @@
                         FontSize="20"
                         WidthRequest="56" HeightRequest="56"
                         CornerRadius="28"
-                        BackgroundColor="{DynamicResource NavigationBarColor}"
-                        TextColor="White"
+                        BackgroundColor="{DynamicResource FabBackgroundColor}"
+                        TextColor="{DynamicResource FabTextColor}"
                         VerticalOptions="End" HorizontalOptions="End"
                         Margin="0,0,16,16"
                         Command="{Binding Commands[AddConnection]}" />

Xamarin.Neo4j/Xamarin.Neo4j/Xamarin.Neo4j/Themes/DarkTheme.xaml

@@ -18,6 +18,8 @@
     <Color x:Key="QueryActionBar">#141414</Color>
     <Color x:Key="Extreme">#141414</Color>
     <Color x:Key="NavigationBarColor">#141414</Color>
+    <Color x:Key="FabBackgroundColor">#ffffff</Color>
+    <Color x:Key="FabTextColor">#0c0c0c</Color>
 
     <Style TargetType="ContentPage" ApplyToDerivedTypes="True">
         <Setter Property="BackgroundColor" Value="{StaticResource PageBackground}" />

Xamarin.Neo4j/Xamarin.Neo4j/Xamarin.Neo4j/Themes/LightTheme.xaml

@@ -18,6 +18,8 @@
     <Color x:Key="QueryActionBar">#f5f5f5</Color>
     <Color x:Key="Extreme">#ffffff</Color>
     <Color x:Key="NavigationBarColor">#0c0c0c</Color>
+    <Color x:Key="FabBackgroundColor">#0c0c0c</Color>
+    <Color x:Key="FabTextColor">#ffffff</Color>
 
     <Style TargetType="ContentPage" ApplyToDerivedTypes="True">
         <Setter Property="BackgroundColor" Value="{StaticResource PageBackground}" />