ui(properties): no raw typography left in the panel, and one heading rank

The earlier pass checked wiring only; this is the typography sweep. Raw Foreground/FontSize
labels are gone from the elements panel:

- Opacity, Timer (minutes) and URI are headings above their control (PropertyHeader), with the
  control row on the panel's 10px indent — the same shape as Border Thickness/Opacity.
- Scroll speed, scoreboard Opacity/Text size and their value readouts are PropertyFieldLabel.
- The drag hint and the scoreboard sign-in note are PropertyHint.
- Simulate (dev) keeps its orange, gains the field-label typography.
- Edge, Text colour and Background were PropertySubHeader/SubHeader: ONE heading rank now, so
  PropertySubHeader and the legacy SubHeader are deleted from Themes/Controls.xaml.
- DeveloperPanel — the last panel with raw typography and an unstyled CheckBox — now uses the
  same styles, and PropertiesStyleTests accepts PropertyPillCheck (the guard went stale when
  every check box became a pill switch). 18 tests pass; Release 0 warnings.

Controls/DeveloperPanel.xaml

@@ -7,15 +7,15 @@
          to a window or reference it from code that compiles in Release; the only host is
          OverlayHost.xaml.cs inside #if DEBUG. -->
     <StackPanel>
-        <TextBlock Text="Developer" Foreground="{StaticResource Brush.Attention}" FontSize="11"
-                   FontWeight="SemiBold" Margin="0,10,0,2"/>
+        <TextBlock Text="Developer" Style="{StaticResource PropertyHeader}"
+                   Foreground="{StaticResource Brush.Attention}"/>
 
-        <CheckBox Margin="10,2,0,0" IsChecked="{Binding DeveloperMode, Mode=TwoWay}"
-                  Foreground="{StaticResource Brush.OnSurface}"
+        <CheckBox Style="{StaticResource PropertyPillCheck}" Margin="10,2,0,0"
+                  IsChecked="{Binding DeveloperMode, Mode=TwoWay}"
                   Content="Developer mode — all resolutions, output budget ignored"/>
 
         <TextBlock Text="Debug builds only. Overrides the free output profile (921,600 px / 30 fps). Never present in a release build."
-                   Foreground="{StaticResource Brush.Attention}" FontSize="10" TextWrapping="Wrap"
+                   Style="{StaticResource PropertyHint}" Foreground="{StaticResource Brush.Attention}"
                    Margin="10,4,0,8"/>
     </StackPanel>
 </UserControl>