fix(dock): size drag preview slots by the edge viewport height

The dock drag preview computed slot heights and drop indices from the full
window height, but docked panel columns only span the model viewport, so the
preview overlay and insertion point drifted below the actual dock edge. Use the
current viewport height so the ghost and drop index line up with the panels.

src/app/update/dialog.rs

@@ -636,6 +636,7 @@ pub(super) fn on_ribbon_tool_click(&mut self, tool_id: String, event: ModuleEven
             }
             DockMsg::DragMove(point) => {
                 if self.dock_dragging.is_some() {
+                    let avail = self.tabs[self.active_tab].scene.selection.borrow().vp_size.1;
                     let side = if point.x < self.win_size.0 * 0.5 {
                         DockSide::Left
                     } else {
@@ -644,7 +645,7 @@ pub(super) fn on_ribbon_tool_click(&mut self, tool_id: String, event: ModuleEven
                     let index = crate::ui::dock::drop_index(
                         point.y,
                         std::cmp::max(self.dock.len(side), 1),
-                        self.win_size.1,
+                        avail,
                     );
                     self.dock_drag_target = Some((side, index));
                 } else if let Some(id) = self.dock_resizing {

src/app/view/mod.rs

@@ -1563,7 +1563,8 @@ impl OpenCADStudio {
             } else {
                 self.dock.len(side) + 1
             };
-            let slot_h = self.win_size.1 / final_count as f32;
+            let edge_h = tab.scene.selection.borrow().vp_size.1;
+            let slot_h = edge_h / final_count as f32;
             let index = self.dock_drag_target.map(|(_, i)| i).unwrap_or(0);
             let slot = index.min(final_count.saturating_sub(1));
             let ghost_top = slot as f32 * slot_h;