feat: A4 landscape default paper, white paper area on dark desk

- paper_limits() falls back to A4 landscape (297×210mm) even when no
  Layout object exists in the document
- paper_bg_color default changed to white [1,1,1,1] (paper surface color)
- PaperCanvas::draw() fills canvas with dark desk color first, then draws
  the paper area rectangle in paper_bg_color so the sheet is clearly
  visible against the desk background
- view.rs container background uses the fixed desk color instead of
  paper_bg_color so it no longer conflicts with the paper-surface color
- BACKGROUND RESET for paper space now resets to white instead of dark gray
- Removed unused PaperSheet variant, paper_sheet() constructor,
  build_paper_sheet_primitive() and associated match arms (dead code
  after 2D canvas replacement)

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

src/app/commands.rs

@@ -41,7 +41,7 @@ impl H7CAD {
                 if args.first().map(|s| s.eq_ignore_ascii_case("RESET")).unwrap_or(false) {
                     if is_paper {
                         self.tabs[i].paper_bg_color = None;
-                        self.tabs[i].scene.paper_bg_color = [0.22, 0.24, 0.28, 1.0];
+                        self.tabs[i].scene.paper_bg_color = [1.0, 1.0, 1.0, 1.0];
                     } else {
                         self.tabs[i].bg_color = None;
                         self.tabs[i].scene.bg_color = [0.11, 0.11, 0.11, 1.0];

src/app/view.rs

@@ -232,9 +232,8 @@ impl H7CAD {
             .height(Fill);
 
         let bg_color = if is_paper {
-            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 })
+            // Desk color — matches the DESK constant in paper_canvas.rs.
+            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 })