fix(ui): open drop_down_below lists right under their field in pallets

iced_aw's DropDown keeps its list on screen by testing it against the
viewport's width and height only, as if the viewport started at the
window corner. Inside a scrollable the viewport is the visible area in
window coordinates, so a field in a dock pallet saw its list overflow
and the list jumped left by its own width (or up over the field); the
Properties Transparency dropdown showed it. Hand the drop-down a
viewport reaching from the corner to the visible area's far edges.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014ihuqzxdUhd9UbioPn5MGu

src/ui/mod.rs

@@ -5,6 +5,7 @@ pub const ROW_H: f32 = 26.0;
 pub mod color_select;
 pub mod command_line;
 pub mod dock;
+pub mod drop_down_fit;
 pub mod hover_zone;
 pub mod icons;
 pub mod menu_bar;