Add focused window controls to top bar

Introduces a new `WindowControls` component in the desktop top bar with minimize, maximize/restore, and close actions for the focused window. `WindowService` now exposes focused-window presence/maximized state plus focused-window convenience actions to support this UI.

Also extends Nebula UI with a reusable `NebulaToolTip`, keyboard/pressed-state support in `NebulaIconButton`, new window-control glyphs in `NebulaIcon`, and theme tokens for pressed/danger hover states, with CMake and README updates to register/document the new components.

packages/nebula-ui/qml/Nebula/UI/NebulaIconButton.qml

@@ -7,6 +7,7 @@ Item {
     property bool active: false
     property bool danger: false
     readonly property bool hovered: hover.containsMouse
+    readonly property bool pressed: hover.pressed
     readonly property color iconColor: {
         if (danger && hovered)
             return Theme.danger
@@ -23,16 +24,32 @@ Item {
 
     implicitWidth: Theme.controlHeight
     implicitHeight: Theme.controlHeight
+    activeFocusOnTab: true
     Accessible.role: Accessible.Button
     Accessible.name: root.label
     Accessible.onPressAction: root.clicked()
 
+    Keys.onReturnPressed: {
+        event.accepted = true
+        root.clicked()
+    }
+    Keys.onSpacePressed: {
+        event.accepted = true
+        root.clicked()
+    }
+
     Rectangle {
         anchors.fill: parent
         radius: Theme.radiusSmall
         color: {
+            if (root.danger && hover.pressed)
+                return Theme.dangerSoftStrong
+            if (root.danger && hover.containsMouse)
+                return Theme.dangerSoft
             if (root.active)
                 return Theme.accentSoft
+            if (hover.pressed)
+                return Theme.pressedFill
             if (hover.containsMouse)
                 return Theme.hoverFill
             return "transparent"
@@ -63,4 +80,9 @@ Item {
     FocusFrame {
         radius: Theme.radiusSmall
     }
+
+    NebulaToolTip {
+        text: root.label
+        active: hover.containsMouse && !hover.pressed && root.label.length > 0
+    }
 }