Add main window with sidebar navigation

AdwNavigationSplitView with six sidebar items (Log, History, Charts,
Budgets, Recurring, Settings) and placeholder StatusPage content that
switches via crossfade transition on selection.

outlay-gtk/src/main.rs

@@ -1,3 +1,5 @@
+mod window;
+
 use adw::prelude::*;
 use adw::Application;
 
@@ -13,12 +15,6 @@ fn main() {
 }
 
 fn build_ui(app: &Application) {
-    let window = adw::ApplicationWindow::builder()
-        .application(app)
-        .title("Outlay")
-        .default_width(900)
-        .default_height(600)
-        .build();
-
-    window.present();
+    let main_window = window::MainWindow::new(app);
+    main_window.window.present();
 }

outlay-gtk/src/window.rs

@@ -0,0 +1,122 @@
+use adw::prelude::*;
+
+pub struct MainWindow {
+    pub window: adw::ApplicationWindow,
+    pub split_view: adw::NavigationSplitView,
+    pub content_stack: gtk::Stack,
+}
+
+struct SidebarItem {
+    id: &'static str,
+    label: &'static str,
+    icon: &'static str,
+}
+
+const SIDEBAR_ITEMS: &[SidebarItem] = &[
+    SidebarItem { id: "log",       label: "Log",       icon: "list-add-symbolic" },
+    SidebarItem { id: "history",   label: "History",   icon: "document-open-recent-symbolic" },
+    SidebarItem { id: "charts",    label: "Charts",    icon: "utilities-system-monitor-symbolic" },
+    SidebarItem { id: "budgets",   label: "Budgets",   icon: "wallet2-symbolic" },
+    SidebarItem { id: "recurring", label: "Recurring", icon: "view-refresh-symbolic" },
+    SidebarItem { id: "settings",  label: "Settings",  icon: "emblem-system-symbolic" },
+];
+
+impl MainWindow {
+    pub fn new(app: &adw::Application) -> Self {
+        let content_stack = gtk::Stack::new();
+        content_stack.set_transition_type(gtk::StackTransitionType::Crossfade);
+
+        for item in SIDEBAR_ITEMS {
+            let page = adw::StatusPage::builder()
+                .title(item.label)
+                .icon_name(item.icon)
+                .build();
+            content_stack.add_named(&page, Some(item.id));
+        }
+
+        let sidebar_list = gtk::ListBox::new();
+        sidebar_list.set_selection_mode(gtk::SelectionMode::Single);
+        sidebar_list.add_css_class("navigation-sidebar");
+
+        for item in SIDEBAR_ITEMS {
+            let row = Self::make_sidebar_row(item);
+            sidebar_list.append(&row);
+        }
+
+        let content_stack_ref = content_stack.clone();
+        sidebar_list.connect_row_selected(move |_, row| {
+            if let Some(row) = row {
+                let idx = row.index() as usize;
+                if idx < SIDEBAR_ITEMS.len() {
+                    content_stack_ref.set_visible_child_name(SIDEBAR_ITEMS[idx].id);
+                }
+            }
+        });
+
+        // Select the first row by default
+        if let Some(first_row) = sidebar_list.row_at_index(0) {
+            sidebar_list.select_row(Some(&first_row));
+        }
+
+        let sidebar_scroll = gtk::ScrolledWindow::builder()
+            .hscrollbar_policy(gtk::PolicyType::Never)
+            .vexpand(true)
+            .child(&sidebar_list)
+            .build();
+
+        let sidebar_toolbar = adw::ToolbarView::new();
+        sidebar_toolbar.add_top_bar(&adw::HeaderBar::new());
+        sidebar_toolbar.set_content(Some(&sidebar_scroll));
+
+        let sidebar_page = adw::NavigationPage::builder()
+            .title("Outlay")
+            .child(&sidebar_toolbar)
+            .build();
+
+        let content_toolbar = adw::ToolbarView::new();
+        content_toolbar.add_top_bar(&adw::HeaderBar::new());
+        content_toolbar.set_content(Some(&content_stack));
+
+        let content_page = adw::NavigationPage::builder()
+            .title("Outlay")
+            .child(&content_toolbar)
+            .build();
+
+        let split_view = adw::NavigationSplitView::new();
+        split_view.set_sidebar(Some(&sidebar_page));
+        split_view.set_content(Some(&content_page));
+
+        let window = adw::ApplicationWindow::builder()
+            .application(app)
+            .title("Outlay")
+            .default_width(900)
+            .default_height(600)
+            .content(&split_view)
+            .build();
+
+        MainWindow {
+            window,
+            split_view,
+            content_stack,
+        }
+    }
+
+    fn make_sidebar_row(item: &SidebarItem) -> gtk::ListBoxRow {
+        let hbox = gtk::Box::new(gtk::Orientation::Horizontal, 12);
+        hbox.set_margin_top(8);
+        hbox.set_margin_bottom(8);
+        hbox.set_margin_start(12);
+        hbox.set_margin_end(12);
+
+        let icon = gtk::Image::from_icon_name(item.icon);
+        let label = gtk::Label::new(Some(item.label));
+        label.set_halign(gtk::Align::Start);
+
+        hbox.append(&icon);
+        hbox.append(&label);
+
+        let row = gtk::ListBoxRow::new();
+        row.set_child(Some(&hbox));
+        row
+    }
+}