fix(ribbon): scrollable layer dropdown, stop mouse leak to viewport (#227)

The ribbon layer dropdown grew unbounded so a long layer list ran off the
bottom of the screen with no way to scroll. Cap the panel height and make
the list scrollable; short lists still shrink to fit.

While a dropdown was open, the 3D viewport beneath it kept tracking the
cursor over the panel's empty areas, and the OS cursor vanished there. In
iced 0.14 mouse_area/opaque only capture button presses, never CursorMoved,
so a higher stack layer can't swallow motion — gate it state-side instead:

- ignore viewport motion in on_viewport_move while ribbon.open_dropdown is set
- suppress the crosshair overlay's Hidden cursor (and crosshair draw) so the
  normal OS cursor shows over an open dropdown
- route all ribbon dropdown backdrops through a shared dropdown_backdrop()

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

src/app/update/viewport.rs

@@ -482,6 +482,15 @@ pub(super) fn on_tick(&mut self, t: Instant) -> Task<Message> {
     }
 
     pub(super) fn on_viewport_move(&mut self, p: Point) -> Task<Message> {
+                // A ribbon dropdown is open over the viewport. Its backdrop
+                // cannot swallow cursor motion — in iced 0.14 mouse_area/opaque
+                // capture only button presses, never CursorMoved — so the move
+                // leaks through the stack to the pane mouse_area beneath and
+                // would track the crosshair over the dropdown's empty areas.
+                // Drop the move here instead. (#227)
+                if self.ribbon.open_dropdown.is_some() {
+                    return Task::none();
+                }
                 let i = self.active_tab;
 
                 // UCS icon grip drag: map the cursor onto the UCS plane and