Add a Drafting page for the rotation angle and the polar increment

Two drafting preferences had no proper home. The Drafting Settings dialog
displays the drafting rotation and offers only a Reset button — there was no
way to type an angle short of `SETVAR SNAPANG`. The polar tracking increment
existed in exactly one place: a pop-up hanging off the status-bar pill.

The rotation is a text field over an edit buffer rather than the value
itself, so clearing it to retype does not snap the crosshair to zero on the
way through; the buffer is reseeded whenever the value is restored from
behind it. Angles render without a trailing `.0`, matching how the polar
pop-up has always shown them — same kind of number, same reader.

Snap modes, grid spacing and object snap stay in Drafting Settings, where
they belong; the page links to it.

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

src/app/update/file.rs

@@ -452,6 +452,9 @@ impl OpenCADStudio {
         } else {
             0.0
         };
+        // The Options field edits a buffer rather than the value, so it has to
+        // be reseeded whenever the value is restored from behind it.
+        self.snap_angle_input = crate::app::settings::format_snap_angle(self.snap_angle_deg);
         // Ortho + running OSNAP are per-drawing (adopted from the header on
         // open / tab switch), not app-global, so they are not applied here.
         self.snapper.otrack_enabled = s.otrack;