fix: NAVVCUBE and UCSICON commands now properly toggle visibility

NAVVCUBE: propagate show_viewcube flag through ViewportPane and the wgpu
Primitive so the ViewCube pipeline skips upload/render when hidden.
UCSICON: bare command (no subcommand) now toggles current state instead
of printing usage info.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/scene/viewport_pane.rs

@@ -25,18 +25,19 @@ pub enum ViewportPaneMode {
 pub struct ViewportPane<'a> {
     pub scene: &'a Scene,
     pub mode: ViewportPaneMode,
+    pub show_viewcube: bool,
 }
 
 impl<'a> ViewportPane<'a> {
-    pub fn model(scene: &'a Scene) -> Self {
-        Self { scene, mode: ViewportPaneMode::Model }
+    pub fn model(scene: &'a Scene, show_viewcube: bool) -> Self {
+        Self { scene, mode: ViewportPaneMode::Model, show_viewcube }
     }
 
     /// One paper-space viewport: model content rendered through its own camera.
     /// See [`ViewportPaneMode::Paper`] for why this is currently unused.
     #[allow(dead_code)]
     pub fn paper(scene: &'a Scene, handle: Handle) -> Self {
-        Self { scene, mode: ViewportPaneMode::Paper { handle } }
+        Self { scene, mode: ViewportPaneMode::Paper { handle }, show_viewcube: false }
     }
 }
 
@@ -106,7 +107,7 @@ impl<'a, Msg: std::fmt::Debug + Clone> shader::Program<Msg> for ViewportPane<'a>
     ) -> Self::Primitive {
         match &self.mode {
             ViewportPaneMode::Model => {
-                self.scene.build_primitive(state.hover_region, bounds)
+                self.scene.build_primitive(state.hover_region, bounds, self.show_viewcube)
             }
             ViewportPaneMode::Paper { handle } => {
                 self.scene.build_viewport_primitive(*handle, state.hover_region, bounds)
@@ -122,8 +123,10 @@ impl<'a, Msg: std::fmt::Debug + Clone> shader::Program<Msg> for ViewportPane<'a>
         cursor: mouse::Cursor,
     ) -> Option<iced::widget::Action<Msg>> {
         // ViewCube hover only makes sense in the full model-space view.
-        if matches!(self.mode, ViewportPaneMode::Model) {
+        if matches!(self.mode, ViewportPaneMode::Model) && self.show_viewcube {
             self.scene.update_viewcube_state(state, bounds, cursor);
+        } else {
+            state.hover_region = None;
         }
         let _ = event;
         None
@@ -135,7 +138,7 @@ impl<'a, Msg: std::fmt::Debug + Clone> shader::Program<Msg> for ViewportPane<'a>
         _b: Rectangle,
         _c: mouse::Cursor,
     ) -> mouse::Interaction {
-        if matches!(self.mode, ViewportPaneMode::Model) {
+        if matches!(self.mode, ViewportPaneMode::Model) && self.show_viewcube {
             self.scene.viewcube_mouse_interaction(state)
         } else {
             mouse::Interaction::default()