feat(gui): KeysPage scaffold + tokio runtime in main, gtk::Box layout

KeysPage::new spawns the connect-and-render flow on glib::MainContext::
spawn_local. On success it shows a placeholder PreferencesPage (Task 5
fills in the list); on failure it shows StatusPage with a Retry button
that re-runs the connect attempt.

main.rs installs the multi-thread tokio runtime guard before app.run()
so zbus's tokio futures execute correctly when polled by glib.

Layout deviation from plan: ToolbarView is libadwaita v1_4-gated; the
project sticks with gtk::Box vertical for portability (commit c6a5e94
established this pattern).

gui/src/main.rs

@@ -3,24 +3,48 @@ use gtk::glib;
 
 mod bus;
 mod error;
+mod keys_page;
 
 const APP_ID: &str = "io.dangerousthings.AuthForge";
 
 fn main() -> glib::ExitCode {
+    // Multi-thread tokio runtime hosts zbus's tokio-flavored futures. The
+    // `_guard` keeps the runtime context active on this thread so calls like
+    // `zbus::Connection::system().await` from within `glib::MainContext::
+    // spawn_local` closures don't panic with "no current reactor".
+    let runtime = tokio::runtime::Builder::new_multi_thread()
+        .enable_all()
+        .build()
+        .expect("build tokio runtime");
+    let _guard = runtime.enter();
+
     let app = adw::Application::builder().application_id(APP_ID).build();
     app.connect_activate(|app| {
-        let status = adw::StatusPage::builder()
-            .icon_name("dialog-password-symbolic")
-            .title("Authentication setup")
-            .description("Phase 0 placeholder. Real UI in Phase 8.")
-            .build();
         let win = adw::ApplicationWindow::builder()
             .application(app)
             .default_width(720)
             .default_height(540)
             .title("Authentication")
-            .content(&status)
             .build();
+        let header = adw::HeaderBar::new();
+        let stack = adw::ViewStack::new();
+        let switcher = adw::ViewSwitcher::builder()
+            .stack(&stack)
+            .policy(adw::ViewSwitcherPolicy::Wide)
+            .build();
+        header.set_title_widget(Some(&switcher));
+
+        let keys = keys_page::KeysPage::new(win.clone().upcast());
+        stack.add_titled(&keys.root, Some("keys"), "Keys");
+
+        // ToolbarView is libadwaita v1_4-gated; the project sticks with the
+        // simpler Box layout for portability (see commit c6a5e94).
+        let layout = gtk::Box::new(gtk::Orientation::Vertical, 0);
+        layout.append(&header);
+        layout.append(&stack);
+        stack.set_vexpand(true);
+
+        win.set_content(Some(&layout));
         win.present();
     });
     app.run()