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 <noreply@anthropic.com>

src/ui/popup/units_popup.rs

@@ -2,7 +2,7 @@
 //! floating overlay above the status bar, same pattern as the scale picker.
 
 use iced::widget::{button, column, container, mouse_area, row, text};
-use iced::{Background, Border, Color, Element, Fill, Length, Padding, Theme};
+use iced::{Background, Border, Color, Element, Fill, Length, Rectangle, Theme};
 
 use crate::app::Message;
 
@@ -37,7 +37,11 @@ pub fn unit_short(code: i16) -> &'static str {
 
 /// Full-screen overlay: transparent click-catcher + units list pinned
 /// bottom-right, above the status bar.
-pub fn units_popup_overlay(current: i16) -> Element<'static, Message> {
+pub fn units_popup_overlay(
+    current: i16,
+    pill: Option<Rectangle>,
+    win: (f32, f32),
+) -> Element<'static, Message> {
     let rows: Vec<Element<'static, Message>> = UNITS
         .iter()
         .map(|&(code, label)| unit_row(label, code == current, Message::SetDrawingUnits(code)))
@@ -55,17 +59,7 @@ pub fn units_popup_overlay(current: i16) -> Element<'static, Message> {
         })
         .width(Length::Fixed(140.0));
 
-    let positioned = container(panel)
-        .align_right(Fill)
-        .align_bottom(Fill)
-        .padding(Padding {
-            bottom: 27.0,
-            right: 4.0,
-            top: 0.0,
-            left: 0.0,
-        })
-        .width(Fill)
-        .height(Fill);
+    let positioned = super::position_statusbar_popup(panel.into(), pill, win, 140.0, true);
 
     mouse_area(positioned)
         .on_press(Message::CloseUnitsPopup)