feat(layout): copy a layout from its tab menu (#1822)

The layout tab's right-click menu gains New Layout, Copy, Page Setup
and Plot beside Rename and Delete. Copy makes "<name> (2)" with the
source's page setup, sheet view and every paper-space object,
viewports included, and opens it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/app/mod.rs

@@ -3765,6 +3765,12 @@ pub enum Message {
     },
     /// Create a new paper space layout.
     LayoutCreate,
+    /// Copy the named layout, its page setup and paper-space objects.
+    LayoutCopy(String),
+    /// Open the named layout's page setup.
+    LayoutPageSetup(String),
+    /// Plot the named layout.
+    LayoutPlot(String),
     /// Delete the named paper space layout (Model cannot be deleted).
     LayoutDelete(String),
     /// Begin inline rename for the given layout tab.