Dock: floating panels, tab groups and adjustable slot heights

Panels can now float over the workspace, share a docked slot as tabs,
and split an edge at heights the user picks.

- Dropping a panel away from the edges floats it at the pointer. Floating
  panels move by their title bar, resize from a corner grip, and come to
  the front when clicked.
- Dropping in the top or bottom quarter of a docked slot docks the panel
  above or below it, as before. Dropping in the middle adds the panel as
  a tab of that slot. Pressing a tab shows that panel; dragging a tab
  pulls the panel out again.
- While dragging, a blue ghost previews the result: the new slot, the
  slot with its future tab row, or the floating window.
- A splitter between stacked slots sets their heights; double-click it
  to make the slots equal again. Heights persist.
- A press only becomes a drag after 5 px of movement, so clicking a title
  bar or tab never moves the panel.

The persisted layout now stores each edge as tab groups with a height
weight, plus a list of floating panels. Configs with the old
bare-panel-id stacks still load as single-panel slots.

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

src/app/node_graph.rs

@@ -173,7 +173,7 @@ impl OpenCADStudio {
                     // The node library docks on the right on first use; after
                     // that it stays wherever the user moved it.
                     let id = crate::ui::dock::PanelId::NodeGraph;
-                    if self.dock.location(id).is_none() {
+                    if !self.dock.is_placed(id) {
                         self.dock
                             .dock(id, crate::app::config::DockSide::Right, usize::MAX);
                     }