fix: correct PageSetup paper size when layout origin is non-zero

paper_limits() can return a non-zero origin (e.g. when min_limits has
negative margin values).  PageSetup was using x1/y1 directly instead of
(x1-x0)/(y1-y0), giving wrong dimensions in those cases.

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

src/app/update.rs

@@ -2668,8 +2668,8 @@ impl H7CAD {
             Message::PageSetupOpen => {
                 let i = self.active_tab;
                 // Populate edit buffers from current paper limits.
-                let (w, h) = if let Some(((_, _), (x1, y1))) = self.tabs[i].scene.paper_limits() {
-                    (x1, y1)
+                let (w, h) = if let Some(((x0, y0), (x1, y1))) = self.tabs[i].scene.paper_limits() {
+                    (x1 - x0, y1 - y0)
                 } else {
                     (297.0, 210.0) // A4 default
                 };