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.

core/windows/WindowBackend.hpp

@@ -0,0 +1,47 @@
+#pragma once
+
+#include "WindowTypes.hpp"
+
+#include <QObject>
+#include <QVector>
+
+class WindowBackend : public QObject
+{
+    Q_OBJECT
+
+public:
+    enum SnapEdge {
+        SnapLeft = 0,
+        SnapRight,
+        SnapMaximize
+    };
+
+    explicit WindowBackend(QObject *parent = nullptr)
+        : QObject(parent)
+    {
+    }
+
+    ~WindowBackend() override = default;
+
+    virtual bool isConnected() const = 0;
+    virtual QVector<WindowInfo> windows() const = 0;
+    virtual QVector<WorkspaceInfo> workspaces() const = 0;
+    virtual qint64 focusedWindowId() const = 0;
+
+    virtual bool focusWindow(qint64 windowId) = 0;
+    virtual bool closeWindow(qint64 windowId) = 0;
+    virtual bool maximizeWindow(qint64 windowId) = 0;
+    virtual bool restoreWindow(qint64 windowId) = 0;
+    virtual bool minimizeWindow(qint64 windowId) = 0;
+    virtual bool setWindowFloating(qint64 windowId, bool floating) = 0;
+    virtual bool moveWindow(qint64 windowId, int x, int y) = 0;
+    virtual bool resizeWindow(qint64 windowId, int width, int height) = 0;
+    virtual bool snapWindow(qint64 windowId, SnapEdge edge) = 0;
+    virtual bool switchWorkspace(const QString &workspaceId) = 0;
+
+signals:
+    void connectedChanged();
+    void windowsChanged();
+    void workspacesChanged();
+    void focusChanged();
+};