fix: Remove default_width, use width_range lower bound as default

Using only width_range(260.0..=600.0) with resizable(true) and no
explicit default_width. The lower bound of the range serves as the default.

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

src/app/file_list.rs

@@ -166,9 +166,6 @@ impl App {
         let show_files = self.config.show_file_panel;
         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| {
                 // The default theme renders unselected list rows fairly dim; bump