Add desktop notifications for budget threshold crossings

After saving an expense, checks if the category budget reaches 75%,
90%, or 100% thresholds. Sends gio::Notification for each newly
crossed threshold. Tracks sent notifications in database to prevent
duplicates within the same month.

outlay-gtk/src/window.rs

@@ -35,7 +35,7 @@ impl MainWindow {
         content_stack.set_transition_type(gtk::StackTransitionType::Crossfade);
 
         // Log view
-        let log_view = LogView::new(db.clone());
+        let log_view = LogView::new(db.clone(), app);
         let log_scroll = gtk::ScrolledWindow::builder()
             .hscrollbar_policy(gtk::PolicyType::Never)
             .child(&log_view.container)