feat(ui): anchor every ribbon dropdown and status-bar popup to its widget, flipping at window edges

All ribbon dropdowns (tool dropdowns, undo/redo history, and the layer /
colour / linetype / lineweight / style combos) and all status-bar popups
(scale, units, OSNAP, isolate, filter, customization menu, layout list)
now open directly at their widget instead of at a layout-estimated
position, and flip to the other side when the panel would cross a window
edge.

Each dropdown/pill widget is wrapped in a transparent PosReport that
records its screen bounds into a thread-local map on draw. The overlays
read those bounds:
- Ribbon: Ribbon::dd_anchor(id, panel_w, win_w) returns
  (align_right, h_pad, top); position_ribbon_dropdown places the panel
  below the widget, flipping right-aligned near the right edge.
- Status bar: popup builders take (pill_bounds, win_size) and call
  popup::position_statusbar_popup / popup_anchor, opening just above the
  pill and flipping left/right at the window edge.

The window size is now tracked (win_size, set on WindowResized). The old
layout estimators compute_dropdown_left / compute_layer_combo_left /
compute_prop_combo_left / compute_history_dropdown_left and
osnap_popup_right_offset are deleted.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/app/mod.rs

@@ -227,6 +227,8 @@ pub(super) struct OpenCADStudio {
     status_bar: StatusBar,
     cursor_pos: Point,
     vp_size: (f32, f32),
+    /// Full window size (w, h); used to anchor status-bar popups to their pill.
+    win_size: (f32, f32),
     snapper: Snapper,
     snap_popup_open: bool,
     scale_popup_open: bool,
@@ -1962,6 +1964,7 @@ impl OpenCADStudio {
             status_bar: StatusBar::new(),
             cursor_pos: Point::ORIGIN,
             vp_size: (1280.0, 720.0),
+            win_size: (1280.0, 720.0),
             snapper: Snapper::default(),
             snap_popup_open: false,
             scale_popup_open: false,