fix: Use if show_files with show() instead of show_animated()

This changes the file list panel to use a conditional if statement with
.show() instead of .show_animated(). This should improve width persistence
by ensuring the panel is only created when visible, avoiding any state
conflicts from show_animated.

Note: This removes the slide animation when showing/hiding the panel.

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 {
             egui::SidePanel::left("files")
                 .resizable(true)
-            // A hard ceiling: the panel is sized from its content, so any row
-            // that asks for more width than there is would otherwise push it
-            // wider every frame. Default width is set via the range's lower bound.
                 .width_range(260.0..=600.0)
-            .show_animated(ctx, show_files, |ui| {
+                .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.
@@ -608,6 +606,7 @@ impl App {
                 }
             });
         }
+    }
 }
 
 /// What a file row's right-click menu asked for. The menu records the choice