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/step_indicator.rs

@@ -1,4 +1,5 @@
 use gtk::prelude::*;
+use gtk::glib::prelude::ToVariant;
 use std::cell::RefCell;
 
 #[derive(Clone)]
@@ -54,8 +55,10 @@ impl StepIndicator {
             let button = gtk::Button::builder()
                 .child(&icon)
                 .has_frame(false)
-                .tooltip_text(format!("Step {}: {}", i + 1, name))
+                .tooltip_text(format!("Step {}: {} (Alt+{})", i + 1, name, i + 1))
                 .sensitive(false)
+                .action_name("win.goto-step")
+                .action_target(&(i as i32 + 1).to_variant())
                 .build();
             button.add_css_class("circular");