feat(viewport): hide ViewCube then render bar as the viewport narrows

When the viewport got too narrow the top-left render-mode bar and the
top-right ViewCube overlapped. Hide them adaptively, measuring the render
bar's real width rather than guessing:

- Wrap the render bar in DensitySwap, which measures its natural width every
  frame and swaps it for an empty spacer once it no longer fits the tile. Add
  report_width0 so it publishes that measured width to render_bar_w.
- Compute viewcube_visible from that measured width: the cube shows only while
  bar_w + gap + cube region fits. This one flag drives the ViewCube nav/UCS
  widgets, the hover hit-test, and — threaded through build_viewports /
  build_viewport_for_pane / viewport_data_for — the GPU cube itself, which
  previously ignored the pane flag (show_viewcube was inst.active only), so
  the cube stayed while its surrounding widgets vanished.

Result: the ViewCube gives way first, then the render bar once it too has no
room — no overlap, sized to the bar's real width (which varies with the
render-mode label, split buttons and tile count).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/mod.rs

@@ -398,6 +398,11 @@ pub(super) struct OpenCADStudio {
     ucs_icon_at_origin: bool,
     /// Whether the ViewCube 3D gizmo is visible in model space (NAVVCUBE).
     show_viewcube: bool,
+    /// Measured natural width (px, as `f32` bits) of the active viewport's
+    /// render-mode control bar, written each frame by its `DensitySwap` and read
+    /// next frame to decide whether the ViewCube still has room beside it — so
+    /// the two corner widgets adapt to the bar's real width, not an estimate.
+    render_bar_w: std::sync::Arc<std::sync::atomic::AtomicU32>,
     /// Whether the Properties panel is shown on the left (PROPERTIES).
     show_properties: bool,
     /// Whether the document file tabs are shown at the top (FILETAB).
@@ -2145,6 +2150,7 @@ impl OpenCADStudio {
             show_ucs_icon: true,
             ucs_icon_at_origin: true,
             show_viewcube: true,
+            render_bar_w: std::sync::Arc::new(std::sync::atomic::AtomicU32::new(0)),
             show_properties: true,
             show_file_tabs: true,
             show_layout_tabs: true,