ui(properties): answer the visual review — quiet actions, real hint tone, dividers, legible disabled

Review findings from a screenshot of the Stream Alerts panel, each addressed:

- "Reset to built-in" and "Browse…" were YtButton-derived (white, semi-bold, 13px), so a reset
  action outranked the section header. Property-panel actions now use PropertyButton: bordered,
  11px, normal weight, OnSurface text.
- Explanation text and field labels were the same muted blue-gray. PropertyHint is now
  OnSurfaceDisabled, one clear step below a label.
- Groups inside a category ran together. PropertyDivider (1px OutlineVariant) separates Video,
  Sound byte and Message ticker.
- The disabled loop switch was near-invisible. Disabled now keeps the label at OnSurfaceVariant
  and dims the whole control to 0.55 — readable, obviously inactive.
- Switch "on" was full-strength accent, shouting about static state. The track now sits at 0.6
  opacity when on, which also stops it competing with the slider's accent fill.

No new palette entries: every change uses an existing token, so DESIGN.md stays the one colour
source. Release 0 warnings; token and typography tests pass.

Themes/Controls.xaml

@@ -177,12 +177,52 @@
     </Style>
 
     <Style x:Key="PropertyHint" TargetType="TextBlock">
-        <Setter Property="Foreground" Value="{StaticResource Brush.OnSurfaceVariant}"/>
+        <Setter Property="Foreground" Value="{StaticResource Brush.OnSurfaceDisabled}"/>
         <Setter Property="FontSize" Value="10"/>
         <Setter Property="TextWrapping" Value="Wrap"/>
         <Setter Property="Margin" Value="10,0,0,6"/>
     </Style>
 
+    <!-- A property-panel action: bordered, quiet, 11px normal weight. The accent-filled YtButton
+         (white semi-bold 13px) made secondary actions like "Reset to built-in" the loudest text in
+         the panel, which is backwards. -->
+    <Style x:Key="PropertyButton" TargetType="Button">
+        <Setter Property="Background" Value="{StaticResource Brush.SurfaceContainerHigh}"/>
+        <Setter Property="Foreground" Value="{StaticResource Brush.OnSurface}"/>
+        <Setter Property="FontSize" Value="11"/>
+        <Setter Property="FontWeight" Value="Normal"/>
+        <Setter Property="Padding" Value="8,3"/>
+        <Setter Property="Cursor" Value="Hand"/>
+        <Setter Property="Template">
+            <Setter.Value>
+                <ControlTemplate TargetType="Button">
+                    <Border x:Name="Bd" Background="{TemplateBinding Background}"
+                            BorderBrush="{StaticResource Brush.Outline}" BorderThickness="1"
+                            CornerRadius="{StaticResource Radius.Sm}"
+                            Padding="{TemplateBinding Padding}">
+                        <ContentPresenter HorizontalAlignment="Center" VerticalAlignment="Center"/>
+                    </Border>
+                    <ControlTemplate.Triggers>
+                        <Trigger Property="IsMouseOver" Value="True">
+                            <Setter TargetName="Bd" Property="Background" Value="{StaticResource Brush.SurfaceContainerHighest}"/>
+                        </Trigger>
+                        <Trigger Property="IsEnabled" Value="False">
+                            <Setter Property="Foreground" Value="{StaticResource Brush.OnSurfaceDisabled}"/>
+                            <Setter TargetName="Bd" Property="Opacity" Value="0.6"/>
+                        </Trigger>
+                    </ControlTemplate.Triggers>
+                </ControlTemplate>
+            </Setter.Value>
+        </Setter>
+    </Style>
+
+    <!-- Hairline between groups inside one properties category. -->
+    <Style x:Key="PropertyDivider" TargetType="Border">
+        <Setter Property="Height" Value="1"/>
+        <Setter Property="Background" Value="{StaticResource Brush.OutlineVariant}"/>
+        <Setter Property="Margin" Value="0,10,0,8"/>
+    </Style>
+
     <!-- Checkboxes are pill switches (creator ruling 2026-11-01): a 14px box in a properties panel
          reads as a form, and a switch reads as a setting. IsChecked semantics are untouched, so
          every existing binding still works. -->
@@ -216,6 +256,8 @@
                         <Trigger Property="IsChecked" Value="True">
                             <Setter TargetName="Track" Property="Background" Value="{StaticResource Brush.Tertiary}"/>
                             <Setter TargetName="Track" Property="BorderBrush" Value="{StaticResource Brush.Tertiary}"/>
+                            <!-- 0.6 keeps the state readable without the neon fill the review flagged. -->
+                            <Setter TargetName="Track" Property="Opacity" Value="0.6"/>
                             <Setter TargetName="Thumb" Property="Fill" Value="{StaticResource Brush.OnAccent}"/>
                             <Trigger.EnterActions>
                                 <BeginStoryboard>
@@ -235,8 +277,10 @@
                             </Trigger.ExitActions>
                         </Trigger>
                         <Trigger Property="IsEnabled" Value="False">
-                            <Setter Property="Foreground" Value="{StaticResource Brush.OnSurfaceDisabled}"/>
-                            <Setter TargetName="Track" Property="Opacity" Value="0.5"/>
+                            <!-- Legible-but-dim, not invisible: the label stays readable and the
+                                 track drops back so "disabled" reads as a state, not as a bug. -->
+                            <Setter Property="Foreground" Value="{StaticResource Brush.OnSurfaceVariant}"/>
+                            <Setter Property="Opacity" Value="0.55"/>
                         </Trigger>
                     </ControlTemplate.Triggers>
                 </ControlTemplate>