feat(polar): angle picker dropdown on the POLAR pill (#264)

The polar-tracking pill only right-click-cycled 15→30→45→90. Add an
AutoCAD-style angle picker opened from a caret on the pill: presets
90/45/30/22.5/18/15/10/5/1 plus a free-text field for any custom angle,
mirroring the existing units / scale status-bar popups. The chosen angle
persists like the other user settings.

Factor the shared chrome into one `split_pill(main, caret_msg, active,
open)` helper (+ `pill_text_color`) so the OSNAP and POLAR pills stop
hand-rolling the same outer border, background, caret and sizing — each
pill now only supplies its own main region. The caret is sized and
aligned to sit inside the single pill border on both.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/update/mod.rs

@@ -1561,6 +1561,37 @@ impl OpenCADStudio {
                 self.polar_increment_deg = deg;
                 self.polar_mode = true;
                 self.ortho_mode = false;
+                self.polar_popup_open = false;
+                Task::none()
+            }
+            Message::TogglePolarPopup => {
+                self.polar_popup_open ^= true;
+                if self.polar_popup_open {
+                    // Start the custom field empty each time; picking a preset or
+                    // typing a value is what actually enables polar tracking.
+                    self.polar_custom_input.clear();
+                }
+                Task::none()
+            }
+            Message::ClosePolarPopup => {
+                self.polar_popup_open = false;
+                Task::none()
+            }
+            Message::PolarCustomInput(s) => {
+                self.polar_custom_input = s;
+                Task::none()
+            }
+            Message::SubmitPolarCustom => {
+                // Accept any positive angle up to a full turn; ignore garbage.
+                if let Ok(v) = self.polar_custom_input.trim().parse::<f32>() {
+                    if v > 0.0 && v <= 360.0 {
+                        self.polar_increment_deg = v;
+                        self.polar_mode = true;
+                        self.ortho_mode = false;
+                    }
+                }
+                self.polar_custom_input.clear();
+                self.polar_popup_open = false;
                 Task::none()
             }
             Message::SetAnnotationScale(scale) => {