feat: per-viewport background color (BACKGROUND command)

- DocumentTab: bg_color / paper_bg_color Optional<[f32;4]> fields
- view.rs: use custom color when set, fall back to defaults
- BACKGROUND <r> <g> <b>: set model or paper-space background (0–255 RGB)
- BACKGROUND RESET: restore default dark grey / paper grey

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

src/app/view.rs

@@ -126,9 +126,13 @@ impl H7CAD {
             .height(Fill);
 
         let bg_color = if is_paper {
-            Color { r: 0.22, g: 0.24, b: 0.28, a: 1.0 }
+            tab.paper_bg_color
+                .map(|[r, g, b, a]| Color { r, g, b, a })
+                .unwrap_or(Color { r: 0.22, g: 0.24, b: 0.28, a: 1.0 })
         } else {
-            Color { r: 0.11, g: 0.11, b: 0.11, a: 1.0 }
+            tab.bg_color
+                .map(|[r, g, b, a]| Color { r, g, b, a })
+                .unwrap_or(Color { r: 0.11, g: 0.11, b: 0.11, a: 1.0 })
         };
 
         let viewport_stack = stack![