fix(viewport): improve cursor and paper contrast

Derive crosshair color from drawing background instead of UI theme.
Use RGB 138 for Paper Space desk background.

Refs #594

src/app/view/mod.rs

@@ -32,6 +32,12 @@ use viewcube::{viewcube_nav_controls, viewcube_ucs_picker, UCS_PICKER_W};
 pub(in crate::app) use overlay::{MTEXT_TEXT_ID, TEXT_INLINE_ID};
 
 const VIEWCUBE_HIT_SIZE: f32 = VIEWCUBE_REGION_PX;
+const PAPER_SPACE_BACKGROUND: Color = Color {
+    r: 138.0 / 255.0,
+    g: 138.0 / 255.0,
+    b: 138.0 / 255.0,
+    a: 1.0,
+};
 
 /// Clear gap (px) kept between the render-mode bar (top-left) and the ViewCube
 /// (top-right) before the cube is judged to collide and hides.
@@ -574,6 +580,11 @@ impl OpenCADStudio {
                 tab.pan_mode,
                 self.ribbon.open_dropdown.is_some(),
                 hover_locked,
+                if tab.scene.input_uses_model_space() {
+                    tab.scene.bg_color
+                } else {
+                    tab.scene.paper_bg_color
+                },
             )
         };
 
@@ -591,13 +602,7 @@ impl OpenCADStudio {
         .on_exit(Message::ViewportExit);
 
         let bg_color = if is_paper {
-            // Desk color — matches the DESK constant in paper_canvas.rs.
+            PAPER_SPACE_BACKGROUND
-            Color {
-                r: 0.22,
-                g: 0.24,
-                b: 0.28,
-                a: 1.0,
-            }
         } else {
             tab.bg_color
                 .map(|[r, g, b, a]| Color { r, g, b, a })
@@ -706,16 +711,10 @@ impl OpenCADStudio {
             // container background, the white sheet + paper entities + borders
             // come from the full-canvas top-locked "sheet" viewport, and the
             // floating content viewports overlay it (same path as model space).
-            const DESK: Color = Color {
-                r: 0.22,
-                g: 0.24,
-                b: 0.28,
-                a: 1.0,
-            };
             stack![
                 container(grid_overlay)
                     .style(move |_: &Theme| container::Style {
-                        background: Some(Background::Color(DESK)),
+                        background: Some(Background::Color(PAPER_SPACE_BACKGROUND)),
                         ..Default::default()
                     })
                     .width(Fill)

src/ui/overlay.rs

@@ -220,6 +220,7 @@ pub fn selection_overlay<'a>(
     pan_mode: bool,
     suppressed: bool,
     hover_locked: bool,
+    crosshair_bg: [f32; 4],
 ) -> Element<'a, Message> {
     canvas(SelectionCanvas {
         selection,
@@ -239,6 +240,7 @@ pub fn selection_overlay<'a>(
         pan_mode,
         suppressed,
         hover_locked,
+        crosshair_bg,
     })
     .width(Length::Fill)
     .height(Length::Fill)
@@ -290,6 +292,9 @@ struct SelectionCanvas {
     /// The entity under the crosshair is on a locked layer — draw a small lock
     /// badge by the cursor so the user knows it can't be selected/edited.
     hover_locked: bool,
+    /// Background of the active drawing space. Crosshair contrast follows this
+    /// rather than the UI theme, which may be light over a dark model viewport.
+    crosshair_bg: [f32; 4],
 }
 
 fn draw_grip_marker(frame: &mut canvas::Frame, grip: &GripMarker, theme: &Theme) {
@@ -908,12 +913,11 @@ impl canvas::Program<Message> for SelectionCanvas {
         // PAN mode replaces the crosshair with a hand cursor.
         if !over_viewcube && !over_divider && !self.pan_mode && !self.suppressed {
             if let Some(cp) = self.selection.last_move_pos {
-                let color = theme
+                let [r, g, b, a] = crate::scene::view::render::adapt_to_bg(
-                    .palette()
+                    [1.0, 1.0, 1.0, 0.90],
-                    .background
+                    self.crosshair_bg,
-                    .base
+                );
-                    .text
+                let color = Color { r, g, b, a };
-                    .scale_alpha(0.90);
                 let stroke = canvas::Stroke {
                     width: 1.0,
                     style: canvas::Style::Solid(color),