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

@@ -164,14 +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;
-        egui::SidePanel::left("files")
-            .resizable(true)
-            .default_width(260.0)
-            // 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.
-            .width_range(160.0..=460.0)
-            .show_animated(ctx, show_files, |ui| {
+        if show_files {
+            egui::SidePanel::left("files")
+                .resizable(true)
+                .width_range(260.0..=600.0)
+                .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.
@@ -251,7 +248,8 @@ impl App {
                 // Measured once, from the panel rather than from the scrolled
                 // content: reading `available_width()` inside a row makes the
                 // content's width depend on the content's width.
-                let row_width = ui.available_width();
+                // Cap the width to prevent the panel from expanding infinitely.
+                let row_width = ui.available_width().min(580.0);
                 let nested = rows.iter().any(|r| r.depth > 0);
 
                 egui::ScrollArea::vertical()
@@ -607,6 +605,7 @@ impl App {
                     });
                 }
             });
+        }
     }
 }