Dock: tab row only for tab groups, larger plain icon tabs, working menu

- The tab row shows only when at least two pallets share a slot.
- Tab icons are larger (18 px). The shown tab gets the grey background
  without a border; other tabs have no background.
- The pallet menu was drawn by a dropdown that squeezed it to one icon
  wide. It is now its own overlay hanging under the menu button, laid
  out like the right-click menu: pallet icon, name, and a check mark on
  the right for the pallets shown in this slot. Any click outside closes
  it.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_013RyGiMXcsf9HTKfqDxaQGt

src/ui/dock.rs

@@ -646,10 +646,10 @@ pub const DOCK_RAIL_W: f32 = 28.0;
 /// Width of the grabbable divider between a docked slot and the viewport.
 pub const DOCK_DIVIDER_W: f32 = 5.0;
 /// Height of a docked slot's tab strip.
-pub const DOCK_TAB_H: f32 = 24.0;
+pub const DOCK_TAB_H: f32 = 28.0;
 
 /// Pitch of one icon tab in a tab strip (tab plus the gap after it).
-pub const DOCK_TAB_CELL_W: f32 = 30.0;
+pub const DOCK_TAB_CELL_W: f32 = 34.0;
 /// Left inset of the first tab in a tab strip.
 pub const DOCK_TAB_INSET: f32 = 4.0;