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/Settings/SettingsSidebar.qml

@@ -648,7 +648,7 @@ Rectangle {
     implicitWidth: __calculatedWidth
     width: __calculatedWidth
     height: parent.height
-    color: Theme.surfaceContainer
+    color: "transparent"
     radius: Theme.cornerRadius
 
     Component.onCompleted: {
@@ -749,12 +749,9 @@ Rectangle {
                 id: searchField
                 width: parent.width - parent.leftPadding - parent.rightPadding
                 placeholderText: I18n.tr("Search...")
-                normalBorderColor: Theme.outlineMedium
-                focusedBorderColor: Theme.primary
                 leftIconName: "search"
                 leftIconSize: Theme.iconSize - 4
                 showClearButton: text.length > 0
-                usePopupTransparency: false
                 onTextChanged: {
                     SettingsSearchService.search(text);
                     root.searchSelectedIndex = 0;