fix: return correct window in relative focus functions

Update SystemWindowRelativeFocus methods to return `currentWindow`
instead of `nil`. This ensures that the current window is 
correctly returned when the focus shift functions are called.

App/Sources/Core/Runners/System/SystemWindowRelativeFocus/SystemWindowRelativeFocusDown.swift

@@ -29,6 +29,6 @@ enum SystemWindowRelativeFocusDown {
       }
     }
 
-    return nil
+    return currentWindow
   }
 }

App/Sources/Core/Runners/System/SystemWindowRelativeFocus/SystemWindowRelativeFocusLeft.swift

@@ -28,7 +28,7 @@ enum SystemWindowRelativeFocusLeft {
       }
     }
 
-    return nil
+    return currentWindow
   }
 }
 

App/Sources/Core/Runners/System/SystemWindowRelativeFocus/SystemWindowRelativeFocusRight.swift

@@ -28,6 +28,6 @@ enum SystemWindowRelativeFocusRight {
       }
     }
 
-    return nil
+    return currentWindow
   }
 }

App/Sources/Core/Runners/System/SystemWindowRelativeFocus/SystemWindowRelativeFocusUp.swift

@@ -29,6 +29,6 @@ enum SystemWindowRelativeFocusUp {
       }
     }
 
-    return nil
+    return currentWindow
   }
 }