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/mod.rs

@@ -1625,6 +1625,10 @@ pub enum Message {
     // ── Ribbon ────────────────────────────────────────────────────────────
     /// User selected a layer from the layer combobox in the ribbon.
     RibbonLayerChanged(String),
+    /// Live text of the ribbon layer dropdown's search box (#343).
+    RibbonLayerFilterChanged(String),
+    /// Live text of the Layer Manager's search box (#343).
+    LayerManagerFilterChanged(String),
     /// User changed the active color in the Properties toolbar.
     RibbonColorChanged(AcadColor),
     /// Toggle the full ACI palette inside the ribbon color picker.