ui(audio): every header label in the sound area uses one colour

MIC's device name was the only header label at full-strength OnSurface (it was brightened
because it shows a choice), and the lip-sync reading was Tertiary — three different values
across four bays in the same row. All four header labels are now Brush.OnSurfaceVariant, the
standard secondary-text token. No other colours touched.

Controls/AudioDock.xaml

@@ -43,7 +43,7 @@
                             </Border>
                             <TextBlock Grid.Column="1" Margin="6,0,0,0" FontSize="11" VerticalAlignment="Center"
                                        TextTrimming="CharacterEllipsis" Text="{Binding MicSourceName, TargetNullValue='No microphone'}"
-                                       Foreground="{StaticResource Brush.OnSurface}"/>
+                                       Foreground="{StaticResource Brush.OnSurfaceVariant}"/>
                         </Grid>
                         <controls:SegmentMeter Grid.Row="1" Height="10" Margin="0,6,0,4" Orientation="Horizontal"
                                                Level="{Binding MicMeterFraction}" Segments="24"/>
@@ -194,7 +194,7 @@
                             </Border>
                             <TextBlock Grid.Column="1" Margin="6,0,0,0" FontSize="10" VerticalAlignment="Center"
                                        FontFamily="{StaticResource FontFamily.Mono}" TextAlignment="Right"
-                                       Foreground="{StaticResource Brush.Tertiary}" Text="{Binding SyncOffsetLabel}"/>
+                                       Foreground="{StaticResource Brush.OnSurfaceVariant}" Text="{Binding SyncOffsetLabel}"/>
                         </Grid>
                         <Border Grid.Row="1" Height="20"/>
                         <StackPanel Grid.Row="2">