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/settings.rs

@@ -145,6 +145,18 @@ pub(crate) fn snaps_from_osmode(osmode: i32) -> (Vec<SnapType>, bool) {
     (modes, osmode & OSMODE_SUPPRESS == 0)
 }
 
+/// Render a drafting angle without a trailing `.0`, so `22.5` but `30`.
+///
+/// The polar pop-up formats its presets the same way; both are showing the
+/// same kind of number to the same person.
+pub fn format_snap_angle(deg: f32) -> String {
+    if (deg - deg.round()).abs() < 1e-4 {
+        format!("{}", deg.round() as i32)
+    } else {
+        format!("{deg}")
+    }
+}
+
 /// GRIPOBJLIMIT default: past this many selected objects, no grips are drawn.
 pub const DEFAULT_GRIP_OBJECT_LIMIT: i32 = 100;