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/commands/blocks.rs

@@ -911,10 +911,9 @@ impl OpenCADStudio {
                 if self.show_external_references {
                     // Always open expanded so the panel is immediately usable;
                     // dock it on the right if the user closed it from the layout.
-                    if self
+                    if !self
                         .dock
-                        .location(crate::ui::dock::PanelId::ExternalReferences)
-                        .is_none()
+                        .is_placed(crate::ui::dock::PanelId::ExternalReferences)
                     {
                         self.dock.dock(
                             crate::ui::dock::PanelId::ExternalReferences,

src/app/commands/count.rs

@@ -236,7 +236,7 @@ impl OpenCADStudio {
         self.count_palette.show = open;
         self.ribbon.set_count_palette(open);
         if open {
-            if self.dock.location(id).is_none() {
+            if !self.dock.is_placed(id) {
                 self.dock.dock(id, crate::app::config::DockSide::Right, usize::MAX);
             }
             self.dock_expanded = Some(id);

src/app/commands/layerprops.rs

@@ -679,7 +679,7 @@ impl OpenCADStudio {
                     // rearrange themselves on upgrade for a panel they have
                     // not asked for.
                     let id = crate::ui::dock::PanelId::Browser;
-                    if self.dock.location(id).is_none() {
+                    if !self.dock.is_placed(id) {
                         // dock() clamps an out-of-range index to the end of
                         // the edge, so this appends below whatever is there.
                         self.dock

src/app/commands/pdf_underlay.rs

@@ -374,7 +374,7 @@ impl OpenCADStudio {
             "POINTCLOUDMANAGER" => {
                 let id = crate::ui::dock::PanelId::PointCloudManager;
                 self.pc_manager.show = true;
-                if self.dock.location(id).is_none() {
+                if !self.dock.is_placed(id) {
                     self.dock.dock(id, crate::app::config::DockSide::Right, usize::MAX);
                 }
                 self.dock_expanded = Some(id);

src/app/commands/sheet_set.rs

@@ -577,7 +577,7 @@ impl OpenCADStudio {
         self.sheet_set.show = on;
         self.ribbon.set_sheet_set(on);
         if on {
-            if self.dock.location(id).is_none() {
+            if !self.dock.is_placed(id) {
                 self.dock.dock(id, crate::app::config::DockSide::Right, usize::MAX);
             }
             self.dock_expanded = Some(id);