fix(common): add missing appearance properties and fix Revealer content detection

Add default values for sizes.elevationMargin, font.family.numbers, and
font.variableAxes.main to prevent undefined property errors when these
aren't specified in user config.

Fix Revealer widget using contentChildren instead of children, which
caused content items to not be detected properly.

quickshell/modules/common/Appearance.qml

@@ -114,9 +114,14 @@ Singleton {
         property int windowRounding: 18
     }
 
+    property QtObject sizes: QtObject {
+        property int elevationMargin: 10
+    }
+
     property QtObject font: QtObject {
         property QtObject family: QtObject {
             property string main: "sans-serif"
+            property string numbers: "sans-serif"
             property string iconMaterial: "Material Symbols Rounded"
         }
         property QtObject pixelSize: QtObject {
@@ -125,6 +130,9 @@ Singleton {
             property int normal: 16
             property int larger: 19
         }
+        property QtObject variableAxes: QtObject {
+            property var main: ({})
+        }
     }
 
     readonly property QtObject animationCurves: QtObject {

quickshell/modules/common/widgets/Revealer.qml

@@ -11,7 +11,7 @@ Item {
     clip: true
 
     // Use first child's implicit size instead of childrenRect to avoid animation feedback loop
-    readonly property Item contentItem: contentChildren.length > 0 ? contentChildren[0] : null
+    readonly property Item contentItem: children.length > 0 ? children[0] : null
     implicitWidth: (reveal || vertical) ? (contentItem?.implicitWidth ?? 0) : 0
     implicitHeight: (reveal || !vertical) ? (contentItem?.implicitHeight ?? 0) : 0
     visible: reveal || (width > 0 && height > 0)