Replace single NavigationView with ViewSwitcher (Installed, Catalog, Updates)

Restructure the app from a single NavigationView into an AdwViewStack
with 3 top-level pages: Installed, Catalog, and Updates. Each page
keeps its own header bar while an AdwViewSwitcherBar at the bottom
provides tab-style navigation between them. The Installed page retains
its own NavigationView for detail drill-down. The catalog action now
switches the ViewStack tab instead of pushing a page.

src/window.rs

@@ -27,6 +27,7 @@ use crate::ui::library_view::{LibraryState, LibraryView};
 use crate::ui::preferences;
 use crate::ui::security_report;
 use crate::ui::update_dialog;
+use crate::ui::updates_view;
 use crate::ui::widgets;
 
 mod imp {
@@ -35,7 +36,8 @@ mod imp {
     pub struct DriftwoodWindow {
         pub settings: OnceCell<gio::Settings>,
         pub toast_overlay: OnceCell<adw::ToastOverlay>,
-        pub navigation_view: OnceCell<adw::NavigationView>,
+        pub view_stack: OnceCell<adw::ViewStack>,
+        pub installed_nav: OnceCell<adw::NavigationView>,
         pub library_view: OnceCell<LibraryView>,
         pub database: OnceCell<Rc<Database>>,
         pub drop_overlay: OnceCell<gtk::Box>,
@@ -48,7 +50,8 @@ mod imp {
             Self {
                 settings: OnceCell::new(),
                 toast_overlay: OnceCell::new(),
-                navigation_view: OnceCell::new(),
+                view_stack: OnceCell::new(),
+                installed_nav: OnceCell::new(),
                 library_view: OnceCell::new(),
                 database: OnceCell::new(),
                 drop_overlay: OnceCell::new(),
@@ -162,9 +165,41 @@ impl DriftwoodWindow {
         // Library view (contains header bar, search, grid/list, empty state)
         let library_view = LibraryView::new(&menu);
 
-        // Navigation view
+        // Installed view: NavigationView for drill-down (detail, dashboard, etc.)
-        let navigation_view = adw::NavigationView::new();
+        let installed_nav = adw::NavigationView::new();
-        navigation_view.push(&library_view.page);
+        installed_nav.push(&library_view.page);
+
+        // Catalog view
+        let catalog_page = catalog_view::build_catalog_page(self.database());
+
+        // Updates view
+        let updates_toolbar = updates_view::build_updates_view(self.database());
+
+        // ViewStack with 3 top-level pages
+        let view_stack = adw::ViewStack::new();
+        view_stack.set_vexpand(true);
+
+        let installed_vs_page = view_stack.add_titled(&installed_nav, Some("installed"), &i18n("Installed"));
+        installed_vs_page.set_icon_name(Some("view-grid-symbolic"));
+
+        let catalog_vs_page = view_stack.add_titled(&catalog_page, Some("catalog"), &i18n("Catalog"));
+        catalog_vs_page.set_icon_name(Some("system-software-install-symbolic"));
+
+        let updates_vs_page = view_stack.add_titled(&updates_toolbar, Some("updates"), &i18n("Updates"));
+        updates_vs_page.set_icon_name(Some("software-update-available-symbolic"));
+
+        // ViewSwitcherBar at the bottom for tab navigation
+        let view_switcher_bar = adw::ViewSwitcherBar::builder()
+            .stack(&view_stack)
+            .reveal(true)
+            .build();
+
+        // Main content box: ViewStack + bottom switcher bar
+        let main_box = gtk::Box::builder()
+            .orientation(gtk::Orientation::Vertical)
+            .build();
+        main_box.append(&view_stack);
+        main_box.append(&view_switcher_bar);
 
         // Drop overlay - centered opaque card over a dimmed scrim
         let drop_overlay_icon = gtk::Image::builder()
@@ -253,9 +288,9 @@ impl DriftwoodWindow {
             drop_overlay_content.add_controller(click);
         }
 
-        // Overlay wraps navigation view so the drop indicator sits on top
+        // Overlay wraps main content so the drop indicator sits on top
         let overlay = gtk::Overlay::new();
-        overlay.set_child(Some(&navigation_view));
+        overlay.set_child(Some(&main_box));
         overlay.add_overlay(&drop_overlay_content);
 
         // Toast overlay wraps the overlay
@@ -353,7 +388,7 @@ impl DriftwoodWindow {
 
         // Wire up card/row activation to push detail view (or toggle selection)
         {
-            let nav = navigation_view.clone();
+            let nav = installed_nav.clone();
             let db = self.database().clone();
             let window_weak = self.downgrade();
             library_view.connect_grid_activated(move |record_id| {
@@ -371,7 +406,7 @@ impl DriftwoodWindow {
             });
         }
         {
-            let nav = navigation_view.clone();
+            let nav = installed_nav.clone();
             let db = self.database().clone();
             let window_weak = self.downgrade();
             library_view.connect_list_activated(move |record_id| {
@@ -394,7 +429,7 @@ impl DriftwoodWindow {
         {
             let db = self.database().clone();
             let window_weak = self.downgrade();
-            navigation_view.connect_popped(move |_nav, page| {
+            installed_nav.connect_popped(move |_nav, page| {
                 if page.tag().as_deref() == Some("detail") {
                     if let Some(window) = window_weak.upgrade() {
                         // Update window title for accessibility (WCAG 2.4.8)
@@ -413,7 +448,7 @@ impl DriftwoodWindow {
         // Update window title when navigating to sub-pages (WCAG 2.4.8 Location)
         {
             let window_weak = self.downgrade();
-            navigation_view.connect_pushed(move |nav| {
+            installed_nav.connect_pushed(move |nav| {
                 if let Some(window) = window_weak.upgrade() {
                     if let Some(page) = nav.visible_page() {
                         let page_title = page.title();
@@ -439,9 +474,13 @@ impl DriftwoodWindow {
             .set(toast_overlay)
             .expect("ToastOverlay already set");
         self.imp()
-            .navigation_view
+            .view_stack
-            .set(navigation_view)
+            .set(view_stack)
-            .expect("NavigationView already set");
+            .expect("ViewStack already set");
+        self.imp()
+            .installed_nav
+            .set(installed_nav)
+            .expect("InstalledNav already set");
         if self.imp().library_view.set(library_view).is_err() {
             panic!("LibraryView already set");
         }
@@ -454,7 +493,7 @@ impl DriftwoodWindow {
         let dashboard_action = gio::ActionEntry::builder("dashboard")
             .activate(|window: &Self, _, _| {
                 let db = window.database().clone();
-                let nav = window.imp().navigation_view.get().unwrap();
+                let nav = window.imp().installed_nav.get().unwrap();
                 let page = dashboard::build_dashboard_page(&db);
                 nav.push(&page);
             })
@@ -554,7 +593,7 @@ impl DriftwoodWindow {
         let security_report_action = gio::ActionEntry::builder("security-report")
             .activate(|window: &Self, _, _| {
                 let db = window.database().clone();
-                let nav = window.imp().navigation_view.get().unwrap();
+                let nav = window.imp().installed_nav.get().unwrap();
                 let page = security_report::build_security_report_page(&db);
                 nav.push(&page);
             })
@@ -568,13 +607,11 @@ impl DriftwoodWindow {
             })
             .build();
 
-        // Catalog browser action
+        // Catalog browser action - switches to catalog tab
         let catalog_action = gio::ActionEntry::builder("catalog")
             .activate(|window: &Self, _, _| {
-                let db = window.database().clone();
+                let view_stack = window.imp().view_stack.get().unwrap();
-                let catalog_page = catalog_view::build_catalog_page(&db);
+                view_stack.set_visible_child_name("catalog");
-                let nav = window.imp().navigation_view.get().unwrap();
-                nav.push(&catalog_page);
             })
             .build();