ui(properties): geometry for every placeable layer, and no fake opacity on a blur

Two holes in the blur's property panel, both of which also affected other layers:

- X/Y/W/H were gated on IsWebcamSelected, so a blur (or an image, chat box or alert zone)
  had no numeric position or size at all — only dragging. The block is generic: it always
  edited SelectedElement's own values, so the gate was the only thing excluding them.
- Opacity showed for a blur, but the compositor's blur reads BlurAmount — the slider would
  have moved a number that changes nothing in the output. Hidden for blur layers.

Blur panel is now: Box | Circle, strength, and the shared geometry fields.

Controls/LeftPanel.xaml

@@ -314,6 +314,7 @@
                             <!-- Opacity + mirror/shape (draggable elements: image, webcam) -->
                             <StackPanel Visibility="{Binding SelectedElement.IsDraggable, Converter={StaticResource BoolToVis}}" Margin="0,0,0,8">
                                 <StackPanel Visibility="{Binding SelectedElement.IsChatBox, Converter={StaticResource InverseBoolToVis}}">
+                                  <StackPanel Visibility="{Binding SelectedElement.IsBlur, Converter={StaticResource InverseBoolToVis}}">
                                     <TextBlock Text="Opacity" Foreground="{StaticResource Brush.OnSurfaceVariant}" FontSize="11" Margin="0,0,0,2"/>
                                     <DockPanel Margin="0,0,0,6">
                                         <TextBlock x:Name="OpacityValueText" Text="100%" Foreground="{StaticResource Brush.OnSurfaceVariant}" FontSize="11"
@@ -322,6 +323,7 @@
                                                 Value="{Binding SelectedElement.Opacity, Mode=TwoWay}"
                                                 ValueChanged="OpacitySlider_ValueChanged" VerticalAlignment="Center"/>
                                     </DockPanel>
+                                  </StackPanel>
                                 </StackPanel>
                                 <!-- TASK 59: the shape is a CHOICE (Rect | Circle) under a
                                      "Shape" title, with Rect selected by default — the circle
@@ -383,7 +385,7 @@
                                  IsWebcamSelected (the same condition as the mirror/shape row
                                  above) so the webcam's border/background section stays the
                                  one panel bound to SelectedElement.IsWebcam. -->
-                            <StackPanel Visibility="{Binding IsWebcamSelected, Converter={StaticResource BoolToVis}}" Margin="0,0,0,8">
+                            <StackPanel Visibility="{Binding SelectedElement.IsDraggable, Converter={StaticResource BoolToVis}}" Margin="0,0,0,8">
                                 <TextBlock Text="Geometry" Foreground="{StaticResource Brush.OnSurfaceVariant}" FontSize="11" FontWeight="SemiBold" Margin="0,8,0,4"/>
                                 <!-- TASK 61: the four fields share ONE fixed width — wide enough
                                      for the largest geometry value (1920, plus a typed sign) and