Add Nebula.Windows with Sway IPC backend

Introduces a new `core/windows` module (`Nebula.Windows`) with `WindowService`, window/workspace models, and a Sway IPC backend for live window tracking and control (focus, close, move/resize, snap, maximize/restore, minimize, workspace switch). The desktop shell now links this module, uses compositor focus for top-bar active app text, and adds an Open Windows section in the launcher with focus/close actions.

Also updates application metadata handling to parse `StartupWMClass` and map window `app_id`/class to friendly names and icons, and revises Sway config/docs for the new floating-first Desktop 0.2 behavior with temporary compositor-provided decorations.

shells/desktop/shell/qml/ShellState.qml

@@ -1,13 +1,14 @@
 pragma Singleton
 
 import QtQuick
+import Nebula.Windows
 
 QtObject {
     property bool launcherOpen: false
 
-    // Focused-window tracking is a later compositor milestone. Launching an
-    // application does not mean it has focus, so this stays "Desktop" for now.
-    property string activeApplication: "Desktop"
+    readonly property string activeApplication: WindowService.focusedApplicationName.length > 0
+        ? WindowService.focusedApplicationName
+        : qsTr("Desktop")
 
     function openLauncher() {
         launcherOpen = true