feat(theme/settings): dedicated floating window styling with shared chrome

- Refactor Widget Styling into Surface Styling & Floating
  Windows
- New sync per-host opacity & foreground layer control
- Shared DankFloatingWindow chrome
- Settings fix to destroy Settings window on-hide so niri floating
  rules match subsequent runs
Related: #2709

quickshell/Modals/Greeter/GreeterFeatureCard.qml

@@ -15,7 +15,7 @@ Rectangle {
 
     height: Math.round(Theme.fontSizeMedium * 6.4)
     radius: Theme.cornerRadius
-    color: Theme.surfaceContainerHigh
+    color: Theme.floatingWindowNestedSurface
 
     Rectangle {
         anchors.fill: parent