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-core/src/db.rs

@@ -470,6 +470,58 @@ impl Database {
         Ok(())
     }
 
+    // -- Budget Notifications --
+
+    pub fn has_notification_been_sent(
+        &self,
+        category_id: i64,
+        month: &str,
+        threshold: u32,
+    ) -> SqlResult<bool> {
+        let count: i64 = self.conn.query_row(
+            "SELECT COUNT(*) FROM budget_notifications
+             WHERE category_id = ?1 AND month = ?2 AND threshold = ?3",
+            params![category_id, month, threshold as i64],
+            |row| row.get(0),
+        )?;
+        Ok(count > 0)
+    }
+
+    pub fn record_notification(
+        &self,
+        category_id: i64,
+        month: &str,
+        threshold: u32,
+    ) -> SqlResult<()> {
+        let now = chrono::Utc::now().to_rfc3339();
+        self.conn.execute(
+            "INSERT OR IGNORE INTO budget_notifications (category_id, month, threshold, notified_at)
+             VALUES (?1, ?2, ?3, ?4)",
+            params![category_id, month, threshold as i64, now],
+        )?;
+        Ok(())
+    }
+
+    pub fn check_budget_thresholds(
+        &self,
+        category_id: i64,
+        month: &str,
+    ) -> SqlResult<Vec<u32>> {
+        let progress = self.get_budget_progress(category_id, month)?;
+        let mut crossed = Vec::new();
+
+        if let Some((_, _, pct)) = progress {
+            for threshold in [75, 90, 100] {
+                if pct >= threshold as f64
+                    && !self.has_notification_been_sent(category_id, month, threshold)?
+                {
+                    crossed.push(threshold);
+                }
+            }
+        }
+        Ok(crossed)
+    }
+
     // -- Exchange Rates --
 
     pub fn get_cached_rate(&self, base: &str, target: &str) -> SqlResult<Option<ExchangeRate>> {

outlay-gtk/src/log_view.rs

@@ -1,5 +1,6 @@
 use adw::prelude::*;
-use gtk::glib;
+use chrono::Datelike;
+use gtk::{gio, glib};
 use outlay_core::db::Database;
 use outlay_core::exchange::ExchangeRateService;
 use outlay_core::models::{NewTransaction, TransactionType};
@@ -12,7 +13,7 @@ pub struct LogView {
 }
 
 impl LogView {
-    pub fn new(db: Rc<Database>) -> Self {
+    pub fn new(db: Rc<Database>, app: &adw::Application) -> Self {
         let toast_overlay = adw::ToastOverlay::new();
         let container = gtk::Box::new(gtk::Orientation::Vertical, 0);
 
@@ -248,6 +249,7 @@ impl LogView {
             let toast_overlay_ref = toast_overlay.clone();
             let rate_ref = exchange_rate.clone();
             let currency_codes_save: Vec<String> = currency_codes.iter().map(|s| s.to_string()).collect();
+            let app_ref = app.clone();
 
             save_button.connect_clicked(move |_| {
                 let amount_text = amount_row_ref.text();
@@ -314,6 +316,29 @@ impl LogView {
                         let toast = adw::Toast::new(msg);
                         toast_overlay_ref.add_toast(toast);
 
+                        // Check budget notifications for expenses
+                        if txn_type == TransactionType::Expense {
+                            let month_str = format!("{:04}-{:02}", date.year(), date.month());
+                            if let Ok(thresholds) = db_ref.check_budget_thresholds(category_id, &month_str) {
+                                let cat_name = db_ref
+                                    .get_category(category_id)
+                                    .map(|c| c.name)
+                                    .unwrap_or_else(|_| "Category".to_string());
+                                let progress = db_ref
+                                    .get_budget_progress(category_id, &month_str)
+                                    .ok()
+                                    .flatten();
+                                let pct = progress.map(|(_, _, p)| p).unwrap_or(0.0);
+
+                                for threshold in thresholds {
+                                    Self::send_budget_notification(
+                                        &app_ref, &cat_name, pct, threshold,
+                                    );
+                                    db_ref.record_notification(category_id, &month_str, threshold).ok();
+                                }
+                            }
+                        }
+
                         amount_row_ref.set_text("");
                         note_row_ref.set_text("");
 
@@ -344,6 +369,26 @@ impl LogView {
         }
     }
 
+    fn send_budget_notification(
+        app: &adw::Application,
+        category: &str,
+        percentage: f64,
+        threshold: u32,
+    ) {
+        let notification = gio::Notification::new("Budget Alert");
+        let body = match threshold {
+            75 => format!("{} is at {:.0}% of budget", category, percentage),
+            90 => format!("{} is at {:.0}% of budget - almost at limit!", category, percentage),
+            100 => format!("{} is over budget at {:.0}%!", category, percentage),
+            _ => return,
+        };
+        notification.set_body(Some(&body));
+        app.send_notification(
+            Some(&format!("budget-{}-{}", category, threshold)),
+            &notification,
+        );
+    }
+
     fn populate_categories_from_db(
         db: &Database,
         model: &gtk::StringList,

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)