feat: Add file panel width persistence to config

- Add file_panel_width field to Config struct with default of 260.0
- Use stored width as default_width for the file list panel
- Update config when panel is resized by dragging separator
- This ensures panel maintains its width across sessions and only
  changes when user explicitly drags the separator

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

src/app/file_list.rs

@@ -165,10 +165,12 @@ impl App {
     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")
+            let panel = egui::SidePanel::left("files")
                 .resizable(true)
-                .width_range(260.0..=600.0)
-                .show(ctx, |ui| {
+                .default_width(self.config.file_panel_width)
+                .width_range(260.0..=600.0);
+            
+            let response = panel.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.
@@ -604,6 +606,13 @@ impl App {
                     });
                 }
             });
+            
+            // Update the config with the current panel width if it changed
+            let new_width = response.response.rect.width();
+            if (new_width - self.config.file_panel_width).abs() > 0.1 {
+                self.config.file_panel_width = new_width;
+                self.config.save();
+            }
         }
     }
 }

src/config.rs

@@ -20,6 +20,9 @@ pub struct Config {
     /// leaves nothing on screen but the page.
     #[serde(default = "default_show_toolbar")]
     pub show_file_panel: bool,
+    /// Width of the left-hand file list panel in pixels.
+    #[serde(default = "default_file_panel_width")]
+    pub file_panel_width: f32,
     /// Whether the editor hides the editorial header, showing the prose alone.
     #[serde(default)]
     pub collapse_header: bool,
@@ -184,6 +187,11 @@ pub fn default_show_toolbar() -> bool {
     true
 }
 
+/// Default width for the file list panel.
+pub fn default_file_panel_width() -> f32 {
+    260.0
+}
+
 /// Default cookiecutter template for **File ▸ New project…**.
 /// Uses the bundled template from the project's templates/snowflake directory.
 pub fn default_project_template() -> PathBuf {
@@ -326,6 +334,7 @@ impl Default for Config {
             show_preview: false,
             show_toolbar: default_show_toolbar(),
             show_file_panel: default_show_toolbar(),
+            file_panel_width: default_file_panel_width(),
             collapse_header: false,
             show_reference_files: false,
             draft_marker: default_marker(),