feat(units): read back every form the drawing writes

The readout learned to write lengths as 5'-6 1/2" and angles as
N 45d0'0" E, but typed input was still a bare parse::<f64>() — a
coordinate could be displayed and not entered. Give the formatters
their inverses.

parse_length takes feet and inches separated by a dash, a space, or
nothing, with the closing quote optional, and whole-plus-fraction in
any of those forms: 5', 60", 5'-9", 5'9-1/2, 9-1/2, 1/2. A leading
minus is a sign; the one inside 5'-9" is a separator, since after feet
there is nothing left to subtract from.

parse_angle reads the mark that says which convention an angle is in —
g for grads, r for radians, d/'/" for degrees-minutes-seconds, compass
letters for a surveyor's bearing — and reads a bare number in whichever
convention the drawing is set to.

ANGBASE and ANGDIR now apply, but only where they mean something. Which
way a thing points is measured from the drawing's zero and runs the way
it says; how wide an arc opens is the same number whatever zero is
counted from. format_direction and parse_direction are the pair for
directions, and the polar readout and polar input both go through them,
so the bearing shown is the bearing accepted. format_angle stays for
sizes, where applying a base angle would corrupt the value.

The command prompts follow the same rule, each classified rather than
swept: OFFSET distance, FILLET radius, CHAMFER distances, ARRAY and
MINSERT spacing, TRACE width, HELIX, and EXTRUDE height read lengths;
the four dimension text-rotation prompts, ARRAY's total angle and
INSERT's rotation read angles. MLINE's scale and INSERT's scale factor
are left alone — a multiplier has no unit to be written in. INSERT read
both through one parse; they are now separate.

The typed variants also take a decimal comma, which a single-value
prompt has no other use for. parse_length itself does not: inside a
coordinate the comma separates the axes.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/app/properties.rs

@@ -89,12 +89,9 @@ impl OpenCADStudio {
         // lengths/angles per LUNITS / LUPREC / AUNITS / AUPREC.
         {
             let h = &self.tabs[i].scene.document.header;
-            crate::entities::common::set_unit_context(crate::entities::common::UnitContext {
-                lunits: h.linear_unit_format,
-                luprec: h.linear_unit_precision,
-                aunits: h.angular_unit_format,
-                auprec: h.angular_unit_precision,
-            });
+            crate::entities::common::set_unit_context(
+                crate::entities::common::UnitContext::from_header(h),
+            );
         }
 
         let layer_names: Vec<String> = self.tabs[i]