Dock: simplify drag, peek, width and open/close state

No intended behaviour change except the fixes noted below.

- One drag state: a single `DockDrag` (move pallet / group, width,
  splitter, floating size, Layer Manager column) replaces nine fields
  (dragging, dragging group, resizing, split, float resizing, origin,
  target, grab offset, layer column flag) that had to be kept in sync.
- `dock_expanded` becomes `dock_peek` with one meaning: a pallet revealed
  although it auto-hides. `dock_reveal(id)` shows a pallet's group and
  peeks it; commands that open a pallet now use it, so opening a pallet
  also switches its edge to that pallet's group.
- Each group stores its own width (falling back to its widest pallet's
  width until first sized), instead of every pallet's width being kept in
  sync. Pallet widths now only size floating pallets and new groups.
- One `dock_set_open(id, open)` holds every pallet's open and close path.
- Leftovers removed: the tab-group config format that never reached main,
  the multi-tab drag ghost header, the separate icon-hover message.

Fixes:
- Hovering a strip icon reveals an auto-hiding edge again (the icons had
  lost the hover message in the option B strip). Covered by a test that
  moves the pointer over the icon in the real view.

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

src/app/node_graph.rs

@@ -177,7 +177,7 @@ impl OpenCADStudio {
                         self.dock
                             .dock(id, crate::app::config::DockSide::Right, usize::MAX);
                     }
-                    self.dock_expanded = Some(id);
+                    self.dock_reveal(id);
                     return self.graph_run(i);
                 }
             }