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/ui/units_popup.rs

@@ -0,0 +1,138 @@
+//! Drawing-units picker — sets the INSUNITS header value. Rendered as a
+//! floating overlay above the status bar, same pattern as the scale picker.
+
+use iced::widget::{button, column, container, mouse_area, row, text};
+use iced::{Background, Border, Color, Element, Fill, Length, Padding, Theme};
+
+use crate::app::Message;
+
+/// Units offered in the picker: (INSUNITS code, menu label).
+const UNITS: &[(i16, &str)] = &[
+    (0, "Unitless"),
+    (4, "Millimeters"),
+    (5, "Centimeters"),
+    (6, "Meters"),
+    (7, "Kilometers"),
+    (1, "Inches"),
+    (2, "Feet"),
+    (3, "Miles"),
+    (10, "Yards"),
+];
+
+/// Short label shown on the status-bar pill for an INSUNITS code.
+pub fn unit_short(code: i16) -> &'static str {
+    match code {
+        1 => "in",
+        2 => "ft",
+        3 => "mi",
+        4 => "mm",
+        5 => "cm",
+        6 => "m",
+        7 => "km",
+        10 => "yd",
+        0 => "Unitless",
+        _ => "Unit",
+    }
+}
+
+/// Full-screen overlay: transparent click-catcher + units list pinned
+/// bottom-right, above the status bar.
+pub fn units_popup_overlay(current: i16) -> Element<'static, Message> {
+    let rows: Vec<Element<'static, Message>> = UNITS
+        .iter()
+        .map(|&(code, label)| unit_row(label, code == current, Message::SetDrawingUnits(code)))
+        .collect();
+
+    let panel = container(column(rows))
+        .style(|_: &Theme| container::Style {
+            background: Some(Background::Color(PANEL_BG)),
+            border: Border {
+                color: PANEL_BORDER,
+                width: 1.0,
+                radius: 3.0.into(),
+            },
+            ..Default::default()
+        })
+        .width(Length::Fixed(140.0));
+
+    let positioned = container(panel)
+        .align_right(Fill)
+        .align_bottom(Fill)
+        .padding(Padding {
+            bottom: 27.0,
+            right: 4.0,
+            top: 0.0,
+            left: 0.0,
+        })
+        .width(Fill)
+        .height(Fill);
+
+    mouse_area(positioned)
+        .on_press(Message::CloseUnitsPopup)
+        .into()
+}
+
+fn unit_row(label: &'static str, active: bool, msg: Message) -> Element<'static, Message> {
+    let check = text(if active { "✓" } else { "  " })
+        .size(11)
+        .color(if active { CHECK_COLOR } else { Color::TRANSPARENT })
+        .width(Length::Fixed(14.0));
+
+    let lbl = text(label)
+        .size(11)
+        .color(if active { LABEL_ON } else { LABEL_OFF });
+
+    let content = row![check, lbl].spacing(6).align_y(iced::Center);
+
+    button(content)
+        .on_press(msg)
+        .style(|_: &Theme, status| button::Style {
+            background: Some(Background::Color(match status {
+                button::Status::Hovered => ROW_HOVER,
+                _ => Color::TRANSPARENT,
+            })),
+            ..Default::default()
+        })
+        .width(Fill)
+        .padding([4, 10])
+        .into()
+}
+
+// ── Colours ───────────────────────────────────────────────────────────────
+
+const PANEL_BG: Color = Color {
+    r: 0.15,
+    g: 0.15,
+    b: 0.15,
+    a: 1.0,
+};
+const PANEL_BORDER: Color = Color {
+    r: 0.32,
+    g: 0.32,
+    b: 0.32,
+    a: 1.0,
+};
+const ROW_HOVER: Color = Color {
+    r: 0.22,
+    g: 0.22,
+    b: 0.22,
+    a: 1.0,
+};
+const CHECK_COLOR: Color = Color {
+    r: 0.35,
+    g: 0.75,
+    b: 1.00,
+    a: 1.0,
+};
+const LABEL_ON: Color = Color {
+    r: 0.92,
+    g: 0.92,
+    b: 0.92,
+    a: 1.0,
+};
+const LABEL_OFF: Color = Color {
+    r: 0.65,
+    g: 0.65,
+    b: 0.65,
+    a: 1.0,
+};