Add GUI crate skeleton with libadwaita placeholder (build deferred until libgtk-4-dev/libadwaita-1-dev installed)

gui/src/main.rs

@@ -0,0 +1,28 @@
+use adw::prelude::*;
+use gtk::glib;
+
+const APP_ID: &str = "io.dangerousthings.UbuntuFido";
+
+fn main() -> glib::ExitCode {
+    let app = adw::Application::builder().application_id(APP_ID).build();
+    app.connect_activate(|app| {
+        let win = adw::ApplicationWindow::builder()
+            .application(app)
+            .default_width(720)
+            .default_height(540)
+            .title("Authentication")
+            .build();
+        let header = adw::HeaderBar::new();
+        let toolbar = adw::ToolbarView::new();
+        toolbar.add_top_bar(&header);
+        let status = adw::StatusPage::builder()
+            .icon_name("dialog-password-symbolic")
+            .title("Authentication setup")
+            .description("Phase 0 placeholder. Real UI in Phase 8.")
+            .build();
+        toolbar.set_content(Some(&status));
+        win.set_content(Some(&toolbar));
+        win.present();
+    });
+    app.run()
+}