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/mod.rs

@@ -655,6 +655,12 @@ pub(super) struct OpenCADStudio {
     /// CATIA navigation: the left button went down while the middle one was
     /// held, so the middle drag orbits until it is released.
     nav_left_with_middle: bool,
+    /// UCSDETECT: a command's first point on a planar solid face sets the
+    /// UCS to that face until the command ends.
+    dynamic_ucs: bool,
+    /// The UCS a dynamic UCS replaced, by tab id, restored when that tab's
+    /// command ends.
+    dynamic_ucs_restore: Option<(u64, Option<codec::tables::Ucs>)>,
     /// Scroll pans and Ctrl+scroll zooms (touchpads); see `scroll_intent`.
     touchpad_pan: bool,
     /// Multi-finger gestures on a touch screen.
@@ -2663,6 +2669,8 @@ pub enum Message {
     /// Toggle reversing the mouse-wheel zoom direction (ZOOMWHEEL).
     ZoomWheelReversedChanged(bool),
     NavigationStyleChanged(crate::app::settings::NavigationStyle),
+    /// F6 / UCSDETECT: toggle the dynamic UCS.
+    ToggleDynamicUcs,
     TouchpadPanChanged(bool),
     /// Options: show the pull-down menus (MENUBAR).
     MenuBarChanged(bool),
@@ -4640,6 +4648,8 @@ impl OpenCADStudio {
             zoom_wheel_reversed: false,
             navigation_style: Default::default(),
             nav_left_with_middle: false,
+            dynamic_ucs: true,
+            dynamic_ucs_restore: None,
             touchpad_pan: false,
             touch: Default::default(),
             touch_offset: None,