Add keyboard focus rings to themed buttons

All Wd.Button.* styles set FocusVisualStyle=x:Null which suppresses WPF's default dotted focus rectangle. With no replacement IsKeyboardFocused trigger, tab-navigating gave NO visual cue — accessibility regression.

Match the keyboard-focus visual to the hover visual so mouse and keyboard land on the same affordance:

- Ghost: cyan border on focus (same as hover, minus the bg fill change so focus + hover compound visibly)

- Caption: lifts to Wd.Button.HoverBg

- RailIcon: lifts to Wd.Button.HoverBg + cyan icon

- IsoToggle: 2px cyan border (same as hover; status bg preserved)

Verified building cleanly. Tab-cycle through the IN-CALL bar / header pills / settings tabs now lights the focused control.

src/TeamsISO.App/Themes/WildDragonTheme.xaml

@@ -167,6 +167,16 @@
                             <Setter TargetName="Bd" Property="Background" Value="{StaticResource Wd.Button.HoverBg}"/>
                             <Setter TargetName="Bd" Property="BorderBrush" Value="{StaticResource Wd.Accent.Cyan}"/>
                         </Trigger>
+                        <!--
+                            Keyboard focus ring (Tab-navigation). FocusVisualStyle
+                            is x:Null on all themed buttons — without this trigger
+                            you'd have NO visual cue when tab-cycling through.
+                            Match the hover treatment so mouse + keyboard land
+                            on the same affordance.
+                        -->
+                        <Trigger Property="IsKeyboardFocused" Value="True">
+                            <Setter TargetName="Bd" Property="BorderBrush" Value="{StaticResource Wd.Accent.Cyan}"/>
+                        </Trigger>
                         <Trigger Property="IsPressed" Value="True">
                             <Setter TargetName="Bd" Property="Background" Value="{StaticResource Wd.Button.PressBg}"/>
                             <Setter TargetName="Bd" Property="BorderBrush" Value="{StaticResource Wd.Accent.CyanHover}"/>
@@ -233,6 +243,9 @@
                         <Trigger Property="IsMouseOver" Value="True">
                             <Setter TargetName="Bd" Property="Background" Value="{StaticResource Wd.Button.HoverBg}"/>
                         </Trigger>
+                        <Trigger Property="IsKeyboardFocused" Value="True">
+                            <Setter TargetName="Bd" Property="Background" Value="{StaticResource Wd.Button.HoverBg}"/>
+                        </Trigger>
                         <Trigger Property="IsPressed" Value="True">
                             <Setter TargetName="Bd" Property="Background" Value="{StaticResource Wd.Button.PressBg}"/>
                         </Trigger>
@@ -290,6 +303,10 @@
                             <Setter TargetName="Bd" Property="Background" Value="{StaticResource Wd.Button.HoverBg}"/>
                             <Setter Property="Foreground" Value="{StaticResource Wd.Accent.Cyan}"/>
                         </Trigger>
+                        <Trigger Property="IsKeyboardFocused" Value="True">
+                            <Setter TargetName="Bd" Property="Background" Value="{StaticResource Wd.Button.HoverBg}"/>
+                            <Setter Property="Foreground" Value="{StaticResource Wd.Accent.Cyan}"/>
+                        </Trigger>
                         <Trigger Property="IsPressed" Value="True">
                             <Setter TargetName="Bd" Property="Background" Value="{StaticResource Wd.Button.PressBg}"/>
                         </Trigger>
@@ -339,6 +356,10 @@
                             <Setter TargetName="Bd" Property="BorderBrush" Value="{StaticResource Wd.Accent.CyanHover}"/>
                             <Setter TargetName="Bd" Property="BorderThickness" Value="2"/>
                         </Trigger>
+                        <Trigger Property="IsKeyboardFocused" Value="True">
+                            <Setter TargetName="Bd" Property="BorderBrush" Value="{StaticResource Wd.Accent.CyanHover}"/>
+                            <Setter TargetName="Bd" Property="BorderThickness" Value="2"/>
+                        </Trigger>
                         <Trigger Property="IsPressed" Value="True">
                             <Setter TargetName="Bd" Property="BorderBrush" Value="{StaticResource Wd.Accent.Cyan}"/>
                             <Setter TargetName="Bd" Property="BorderThickness" Value="2"/>