Initialize overwrite behavior from app settings

The overwrite behavior in the output step now defaults to the value
configured in Settings rather than always defaulting to "Ask".

pixstrip-gtk/src/app.rs

@@ -288,7 +288,12 @@ fn build_ui(app: &adw::Application) {
             rename_counter_padding: 3,
             rename_template: String::new(),
             preserve_dir_structure: false,
-            overwrite_behavior: 0,
+            overwrite_behavior: match app_cfg.overwrite_behavior {
+                pixstrip_core::config::OverwriteBehavior::Ask => 0,
+                pixstrip_core::config::OverwriteBehavior::AutoRename => 1,
+                pixstrip_core::config::OverwriteBehavior::Overwrite => 2,
+                pixstrip_core::config::OverwriteBehavior::Skip => 3,
+            },
         })),
     };