feat: Add file panel width persistence to config

- Add file_panel_width field to Config struct with default of 260.0
- Use stored width as default_width for the file list panel
- Update config when panel is resized by dragging separator
- This ensures panel maintains its width across sessions and only
  changes when user explicitly drags the separator

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

src/app/file_list.rs

@@ -165,10 +165,12 @@ impl App {
     pub(super) fn left_pane(&mut self, ctx: &egui::Context) {
         let show_files = self.config.show_file_panel;
         if show_files {
-            egui::SidePanel::left("files")
+            let panel = egui::SidePanel::left("files")
                 .resizable(true)
-                .width_range(260.0..=600.0)
-                .show(ctx, |ui| {
+                .default_width(self.config.file_panel_width)
+                .width_range(260.0..=600.0);
+            
+            let response = panel.show(ctx, |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.
@@ -604,6 +606,13 @@ impl App {
                     });
                 }
             });
+            
+            // Update the config with the current panel width if it changed
+            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;
+                self.config.save();
+            }
         }
     }
 }