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/ui/popup/mod.rs

@@ -1,5 +1,6 @@
 pub mod cycle_popup;
 pub mod isolate_popup;
+pub mod polar_popup;
 pub mod scale_popup;
 pub mod selection_filter_popup;
 pub mod snap_popup;