fix: Use show_animated with width persistence for file panel

- Use show_animated instead of show() to maintain egui's animation
- Add .default_width() from config
- Update config when panel width changes
- This should allow the panel to maintain its width after resizing

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

src/app/file_list.rs

@@ -164,13 +164,11 @@ impl App {
     /// editor rather than leaving an empty column.
     pub(super) fn left_pane(&mut self, ctx: &egui::Context) {
         let show_files = self.config.show_file_panel;
-        if show_files {
-            let panel = egui::SidePanel::left("files")
-                .resizable(true)
-                .default_width(self.config.file_panel_width)
-                .width_range(260.0..=600.0);
-            
-            let response = panel.show(ctx, |ui| {
+        if let Some(response) = egui::SidePanel::left("files")
+            .resizable(true)
+            .default_width(self.config.file_panel_width)
+            .width_range(260.0..=600.0)
+            .show_animated(ctx, show_files, |ui| {
                 // The default theme renders unselected list rows fairly dim; bump
                 // the widget text colours so file names stay legible (especially in
                 // dark mode) without affecting the rest of the app.
@@ -605,9 +603,8 @@ impl App {
                         }
                     });
                 }
-            });
-            
-            // Update the config with the current panel width if it changed
+            }) {
+            // Update the config with the current panel width
             let new_width = response.response.rect.width();
             if (new_width - self.config.file_panel_width).abs() > 0.1 {
                 self.config.file_panel_width = new_width;