Layouts: a viewport on first opening (LAYOUTCREATEVIEWPORT)

A layout opened for the first time - one whose sheet viewport had to be
made, or one created in this session - gets a viewport centred on the
printable area, 0.8 of its width and height, zoomed to the model's
extents, while LAYOUTCREATEVIEWPORT is on. The profile variable takes
ON/OFF or 1/0 and is kept with the settings.

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

src/app/settings.rs

@@ -423,6 +423,9 @@ pub struct UserSettings {
     pub ucs_ortho: bool,
     /// UCSVIEW: a saved named view keeps the current UCS.
     pub ucs_view: bool,
+    /// LAYOUTCREATEVIEWPORT: a layout opened for the first time gets a
+    /// viewport.
+    pub layout_create_viewport: bool,
     /// Nested-copy symbol handling: false inserts, true binds.
     pub ncopy_bind: bool,
     /// Last Options page; unknown saved names fall back without rejecting the config.
@@ -842,6 +845,7 @@ impl Default for UserSettings {
             surface_associativity: true,
             ucs_ortho: true,
             ucs_view: true,
+            layout_create_viewport: true,
             ncopy_bind: false,
             cursor_type: CursorType::Crosshair,
             crosshair_color: None,