feat(statusbar): drawing-units pill with INSUNITS picker

Add a Units pill that shows the drawing's current INSUNITS value and
opens a picker to change it (Unitless, mm, cm, m, km, in, ft, mi, yd).
Toggleable from the status-bar customization menu like the other pills.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/mod.rs

@@ -173,6 +173,8 @@ pub(super) struct OpenCADStudio {
     scale_popup_open: bool,
     /// True while the status-bar customization menu is open.
     statusbar_menu_open: bool,
+    /// True while the drawing-units picker is open.
+    units_popup_open: bool,
     /// Clean-screen mode: hide ribbon and side panels for a full canvas.
     clean_screen: bool,
     /// Which status-bar pills the user has chosen to show (persisted).
@@ -746,6 +748,12 @@ pub enum Message {
     ToggleStatusPill(crate::ui::statusbar_config::StatusPill),
     /// Toggle clean-screen mode (hide ribbon + side panels).
     ToggleCleanScreen,
+    /// Toggle the drawing-units picker open/closed.
+    ToggleUnitsPopup,
+    /// Close the drawing-units picker.
+    CloseUnitsPopup,
+    /// Set the drawing units (INSUNITS) for the active drawing.
+    SetDrawingUnits(i16),
     /// Toggle dynamic input overlay (F12).
     ToggleDynInput,
     /// Toggle object snap tracking (F11).
@@ -1203,6 +1211,7 @@ impl OpenCADStudio {
             snap_popup_open: false,
             scale_popup_open: false,
             statusbar_menu_open: false,
+            units_popup_open: false,
             statusbar_config: crate::ui::statusbar_config::StatusBarConfig::load(),
             clean_screen: false,
             pre_cmd_tangent: None,

src/app/update.rs

@@ -3580,6 +3580,21 @@ impl OpenCADStudio {
                 self.clean_screen ^= true;
                 Task::none()
             }
+            Message::ToggleUnitsPopup => {
+                self.units_popup_open ^= true;
+                Task::none()
+            }
+            Message::CloseUnitsPopup => {
+                self.units_popup_open = false;
+                Task::none()
+            }
+            Message::SetDrawingUnits(code) => {
+                self.units_popup_open = false;
+                let i = self.active_tab;
+                self.tabs[i].scene.document.header.insertion_units = code;
+                self.tabs[i].dirty = true;
+                Task::none()
+            }
             Message::ToggleSnap(t) => {
                 self.snapper.toggle(t);
                 Task::none()

src/app/view.rs

@@ -1157,6 +1157,8 @@ impl OpenCADStudio {
                         tab.scene.document.header.lineweight_display,
                         cursor_coord,
                         self.clean_screen,
+                        tab.scene.document.header.insertion_units,
+                        self.units_popup_open,
                         &self.statusbar_config,
                     )
                 })
@@ -1199,6 +1201,14 @@ impl OpenCADStudio {
             iced::widget::Space::new().width(0).height(0).into()
         };
 
+        let units_layer: Element<'_, Message> = if self.units_popup_open {
+            crate::ui::units_popup::units_popup_overlay(
+                tab.scene.document.header.insertion_units,
+            )
+        } else {
+            iced::widget::Space::new().width(0).height(0).into()
+        };
+
         let dropdown_layer: Element<'_, Message> = self
             .ribbon
             .dropdown_overlay(
@@ -1235,6 +1245,7 @@ impl OpenCADStudio {
             snap_layer,
             scale_layer,
             statusbar_menu_layer,
+            units_layer,
             dropdown_layer,
             layout_ctx_layer,
             qselect_layer,