fix: Cap row_width to prevent panel from expanding infinitely

Capped row_width at 580.0 to prevent the content from pushing the
panel wider than the user intends, breaking the content-width loop.

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

src/app/file_list.rs

@@ -250,7 +250,8 @@ 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()