fix(dock): keep auto-hiding panels shown while the pointer is on their dropdowns

iced gives the layer under an overlay a levitating cursor, so the dock
edge's mouse_area read the pointer on any dropdown or combo list in a
pallet as leaving, and auto-hide collapsed the panel mid-pick. Track
enter/leave with a small hover_zone wrapper that lands the cursor before
its bounds test, on docked edges and floating pallets alike.

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 hover_zone;
 pub mod icons;
 pub mod menu_bar;
 pub mod modal;