fix(units): make the units selector reach what it names

The status-bar units button wrote INSUNITS, which nothing on screen reads.
INSUNITS only tells the drawing how to scale content arriving from
elsewhere, so picking a unit relabelled the drawing and changed nothing
visible — the selector looked broken because the variable it moved was
invisible.

Point the button at LUNITS instead, the setting that decides how every
coordinate and distance is written, and give the three questions the word
"units" runs together a place each:

- The button offers the five length formats, each shown beside a sample of
  itself, so Engineering and Architectural are told apart by sight.
- UNITS / DDUNITS opens a real dialog: length and angle format and
  precision, the direction angle zero points, whether positive angles run
  clockwise, and the insertion unit. Its sample comes from the same
  formatters the drawing uses, so it cannot drift from the readout.
- DWGUNITS converts. Model geometry, viewport framing, the header's length
  defaults and each dimension style's scale all move together in one undo
  step; paper space is left alone, since the sheet is still the same sheet.
  Locked layers are converted too — a half-converted drawing would be at
  two scales at once.

The coordinate readout formatted at four decimal places whatever the
drawing asked for; it now goes through the same format_length the
properties panel uses. That made the angle formats reachable in turn, so
degrees-minutes-seconds now carries the d/'/" marks that say which
convention it is, and surveyor's units are written at all.

Typed input is still plain decimal: a length the readout writes as
5'-6 1/2" cannot yet be typed back. ANGBASE and ANGDIR are stored but not
yet applied to displayed directions.

Closes #668

Co-Authored-By: Claude Opus 5 <[email protected]>

src/app/mod.rs

@@ -408,6 +408,8 @@ pub(super) struct OpenCADStudio {
     last_layer_translation: Option<crate::modules::draw::layers::laytrans::Report>,
     /// The layer-translator dialog's working state while it is open.
     layer_translator: Option<crate::ui::window::layer_translator::State>,
+    /// Working copy of the Drawing Units dialog; `None` while it is closed.
+    drawing_units: Option<crate::ui::window::drawing_units::State>,
     /// PICKDRAG (#226): false (default) = press-drag lassoes; true =
     /// press-drag draws a rectangle marquee.
     pick_drag_rect: bool,
@@ -1495,6 +1497,7 @@ pub enum ModalKind {
     Layers,
     LayerStateManager,
     LayerTranslator,
+    DrawingUnits,
     LayerStateEditor,
     Plot,
     PrintAll,
@@ -2196,6 +2199,14 @@ pub enum Message {
     CloseUnitsPopup,
     /// Set the drawing units (INSUNITS) for the active drawing.
     SetDrawingUnits(i16),
+    /// LUNITS — how lengths are written, from the status-bar units button.
+    SetLinearFormat(i16),
+    /// Open the Drawing Units dialog, seeded from the active drawing.
+    OpenDrawingUnits,
+    /// One field of the Drawing Units dialog changed.
+    DrawingUnitsField(crate::ui::window::drawing_units::Field),
+    /// Drawing Units OK — write the working copy into the drawing.
+    DrawingUnitsApply,
     /// Toggle the Isolate pill's action menu open/closed.
     ToggleIsolatePopup,
     /// Close the Isolate action menu.
@@ -3009,6 +3020,7 @@ impl OpenCADStudio {
             select_remove_mode: false,
             last_layer_translation: None,
             layer_translator: None,
+            drawing_units: None,
             pick_drag_rect: false,
             perf_hud: false,
             cycle_candidates: None,