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>> {