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();
 }