feat(layers): search boxes in the layer dropdown and Layer Manager (#343)

The ribbon layer dropdown gets a live search field above the list
(cleared when the dropdown closes, panel height tracks the filtered
count), and the Layer Manager toolbar gets one too. Both filter by
case-insensitive substring while toggle/rename messages keep the
original row indices, so acting on a filtered row hits the right layer.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/app/update/mod.rs

@@ -3246,6 +3246,15 @@ impl OpenCADStudio {
                 self.modal_drag_last = None;
                 Task::none()
             }
+            Message::RibbonLayerFilterChanged(f) => {
+                self.ribbon.layer_filter = f;
+                Task::none()
+            }
+            Message::LayerManagerFilterChanged(f) => {
+                let i = self.active_tab;
+                self.tabs[i].layers.filter = f;
+                Task::none()
+            }
             Message::LayerNameColGrab => {
                 // Start a Name-column divider drag; rides ModalDragMove.
                 self.layer_col_dragging = true;