Add window persistence, clickable step indicator, file list improvements

- Window size/position remembered between sessions via SessionStore
- Step indicator dots now clickable to navigate directly to that step,
  with keyboard shortcut hints in tooltips
- File list in add-files dialog shows format and size per image,
  header shows total count and total size
- Welcome dialog now saves skill level choice to config
- SessionState extended with window_width, window_height, window_maximized

pixstrip-gtk/src/welcome.rs

@@ -138,7 +138,18 @@ fn build_skill_page(nav_view: &adw::NavigationView) -> adw::NavigationPage {
     next_button.add_css_class("pill");
 
     let nav = nav_view.clone();
+    let sc = simple_check.clone();
     next_button.connect_clicked(move |_| {
+        // Save skill level choice
+        let store = pixstrip_core::storage::ConfigStore::new();
+        if let Ok(mut cfg) = store.load() {
+            cfg.skill_level = if sc.is_active() {
+                pixstrip_core::config::SkillLevel::Simple
+            } else {
+                pixstrip_core::config::SkillLevel::Detailed
+            };
+            let _ = store.save(&cfg);
+        }
         nav.push_by_tag("output-location");
     });