Add GTK app shell with wizard navigation, step indicator, and actions

7-step wizard flow (Workflow, Images, Resize, Convert, Compress,
Metadata, Output) with AdwNavigationView, step indicator dots,
Back/Next buttons, keyboard shortcuts (Alt+arrows, Alt+1-9),
and hamburger menu with Settings and History placeholders.

pixstrip-gtk/src/main.rs

@@ -1,38 +1,10 @@
-use adw::prelude::*;
+mod app;
+mod step_indicator;
+mod wizard;
 
-const APP_ID: &str = "live.lashman.Pixstrip";
+use gtk::prelude::*;
 
 fn main() {
-    let app = adw::Application::builder()
-        .application_id(APP_ID)
-        .build();
-
-    app.connect_activate(build_ui);
+    let app = app::build_app();
     app.run();
 }
-
-fn build_ui(app: &adw::Application) {
-    let header = adw::HeaderBar::new();
-
-    let title = adw::WindowTitle::new("Pixstrip", "Batch Image Processor");
-    header.set_title_widget(Some(&title));
-
-    let content = adw::StatusPage::builder()
-        .title("Pixstrip")
-        .description(format!("v{}", pixstrip_core::version()).as_str())
-        .icon_name("image-x-generic-symbolic")
-        .build();
-
-    let toolbar_view = adw::ToolbarView::new();
-    toolbar_view.add_top_bar(&header);
-    toolbar_view.set_content(Some(&content));
-
-    let window = adw::ApplicationWindow::builder()
-        .application(app)
-        .default_width(900)
-        .default_height(650)
-        .content(&toolbar_view)
-        .build();
-
-    window.present();
-}