Dock: Layer Manager pallet, double-click to float / dock

- LAYERS opens the Layer Manager as a dockable pallet instead of a modal
  dialog with a darkened backdrop. It floats over the middle of the
  drawing the first time and keeps its place after that; LAYERS again
  (or its close button) hides it. The table keeps its natural width and
  scrolls sideways when the pallet is narrower, and the Name column
  divider now drags through the dock's pointer capture. Ctrl+A selects
  all layers while a layer row is selected in the open pallet.
- The Layer Manager is listed in the tab-row pallet menu like the others.
- Double-clicking a docked title bar floats the panel beside its edge;
  double-clicking a floating title bar docks the panel on the side its
  title bar faces.

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

src/app/mod.rs

@@ -814,6 +814,8 @@ pub(super) struct OpenCADStudio {
     render_mode_preview: Option<codec::entities::ViewportRenderMode>,
     /// Whether the Properties panel is shown on the left (PROPERTIES).
     show_properties: bool,
+    /// The Layer Manager pallet (LAYERS) is open.
+    pub(crate) show_layers: bool,
     /// Docked Insert Block panel visibility.
     pub(crate) show_block_palette: bool,
     /// Node graph overlay over the viewport.
@@ -1963,7 +1965,6 @@ pub enum ModalKind {
     PluginManager,
     UpdateNotice,
     DonationPrompt,
-    Layers,
     LayerStateManager,
     LayerTranslator,
     DrawingUnits,
@@ -4285,6 +4286,7 @@ impl OpenCADStudio {
             render_mode_menu_open: false,
             render_mode_preview: None,
             show_properties: true,
+            show_layers: false,
             show_block_palette: false,
             show_node_graph: false,
             property_target_override: None,