Dock: floating windows hold one pallet again

Remove floating groups: a floating window is a single pallet once more,
and the Pallets submenu is gone from its title-bar menu. Kept: the menu
opens at the pointer with Allow docking, Dock left and Dock right;
close / pin show only while the bar is hovered; content is clipped to
its frame. A config saved by the floating-group build still loads, its
windows floating their first pallet. Drops the unused Pallets string.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013RyGiMXcsf9HTKfqDxaQGt

src/app/mod.rs

@@ -865,10 +865,8 @@ pub(super) struct OpenCADStudio {
     pub(crate) dock_title_hover: Option<crate::ui::dock::PanelId>,
     /// Edge whose icon-strip pallet menu (+) is open.
     pub(crate) dock_edge_menu: Option<crate::app::config::DockSide>,
-    /// Floating window (named by one of its pallets) whose right-click menu
-    /// is open, and whether its Pallets submenu shows.
+    /// Floating pallet whose title-bar right-click menu is open.
     pub(crate) dock_float_menu: Option<crate::ui::dock::PanelId>,
-    pub(crate) dock_float_menu_pallets: bool,
     /// Pointer position over a floating title bar (bar-local), and where
     /// its open menu was asked for.
     pub(crate) dock_float_pointer: iced::Point,
@@ -4306,7 +4304,6 @@ impl OpenCADStudio {
             dock_hover_pending: None,
             dock_hover_gen: 0,
             dock_float_menu: None,
-            dock_float_menu_pallets: false,
             dock_float_pointer: iced::Point::ORIGIN,
             dock_float_menu_at: iced::Point::ORIGIN,
             dock_drag: None,