ucs: dynamic UCS on planar solid faces

With dynamic UCS on (UCSDETECT 1, toggled with F6), a command's first
point picked on a planar face of a solid sets the UCS to that face for
the rest of the command, so 2D commands draw on the face (#1711). An
unsnapped first point lands on the face itself; a snapped point off the
face's plane belongs to other geometry and leaves the UCS alone. The
previous UCS comes back when the command ends. UCS commands themselves
are left out.

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

src/app/settings.rs

@@ -416,6 +416,9 @@ pub struct UserSettings {
     pub polar_increment_deg: f32,
     pub zoom_wheel_reversed: bool,
     pub navigation_style: NavigationStyle,
+    /// UCSDETECT: a command's first point on a planar solid face sets the
+    /// UCS to that face until the command ends.
+    pub dynamic_ucs: bool,
     /// Scrolling pans and Ctrl+scroll (a touchpad pinch) zooms, for a
     /// touchpad that reports two-finger drags as wheel scrolls.
     pub touchpad_pan: bool,
@@ -920,6 +923,7 @@ impl Default for UserSettings {
             polar_increment_deg: 45.0,
             zoom_wheel_reversed: false,
             navigation_style: NavigationStyle::Cad,
+            dynamic_ucs: true,
             touchpad_pan: false,
             ui_scale_percent: 100,
             zoom_factor: 60,