feat(layers): draggable Name-column width in the Layer Manager (#359)

A thin divider after the Name header resizes the column by drag
(60-640 px, session-persistent), riding the shared modal drag flow.
The name elide budget follows the column width, so widening the
column reveals long layer names; the hover tooltip stays for overflow.

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

src/app/view/modal.rs

@@ -81,7 +81,7 @@ impl OpenCADStudio {
             }
             super::super::ModalKind::Layers => {
                 let tab = &self.tabs[self.active_tab];
-                sized(tab.layers.view_window(), 900, 360)
+                sized(tab.layers.view_window(self.layer_name_col_w), 900, 360)
             }
             super::super::ModalKind::Plot => {
                 sized(crate::ui::window::plot::view_window(&self.plot_dialog), 760, 540)