Revert: Set file list width range to 160-600 (was INFINITY)

The INFINITY upper bound caused the panel to expand to fill the window.
Changed back to a reasonable max of 600.0 pixels.

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

src/app/file_list.rs

@@ -169,9 +169,8 @@ impl App {
             .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. We set a minimum width but no maximum to
-            // allow the user to make the panel as wide as they want.
-            .width_range(160.0..=f32::INFINITY)
+            // wider every frame.
+            .width_range(160.0..=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