Add feature batch 2, subscription/recurring sync, smooth charts, and app icon

- Implement subscriptions view with bidirectional recurring transaction sync
- Add cascade delete/pause/resume between subscriptions and recurring
- Fix foreign key constraints when deleting recurring transactions
- Add cross-view instant refresh via callback pattern
- Replace Bezier chart smoothing with Fritsch-Carlson monotone Hermite interpolation
- Smooth budget sparklines using shared monotone_subdivide function
- Add vertical spacing to budget rows
- Add app icon (receipt on GNOME blue) in all sizes for desktop, web, and AppImage
- Add calendar, credit cards, forecast, goals, insights, and wishlist views
- Add date picker, numpad, quick-add, category combo, and edit dialog components
- Add import/export for CSV, JSON, OFX, QIF formats
- Add NLP transaction parsing, OCR receipt scanning, expression evaluator
- Add notification support, Sankey chart, tray icon
- Add demo data seeder with full DB wipe
- Expand database schema with subscriptions, goals, credit cards, and more

outlay-core/src/notifications.rs

@@ -0,0 +1,76 @@
+use crate::db::Database;
+use std::process::Command;
+
+/// Send a desktop notification via notify-send (Linux).
+/// Returns silently if notify-send is not available.
+pub fn send_notification(title: &str, body: &str, urgency: &str) {
+    let _ = Command::new("notify-send")
+        .arg("--urgency")
+        .arg(urgency)
+        .arg("--app-name=Outlay")
+        .arg(title)
+        .arg(body)
+        .spawn();
+}
+
+/// Check all budgets for the given month and send notifications
+/// for any thresholds crossed that haven't been notified yet.
+/// Only sends if budget_notifications setting is enabled.
+pub fn check_and_send_budget_notifications(db: &Database, month: &str) {
+    let enabled = db.get_setting("budget_notifications")
+        .ok().flatten().map(|s| s == "1").unwrap_or(false);
+    if !enabled {
+        return;
+    }
+
+    let budgets = match db.list_budgets_for_month(month) {
+        Ok(b) => b,
+        Err(_) => return,
+    };
+
+    for budget in &budgets {
+        let cat_name = db.get_category(budget.category_id)
+            .map(|c| c.name)
+            .unwrap_or_else(|_| "Unknown".to_string());
+
+        let thresholds = match db.check_budget_thresholds(budget.category_id, month) {
+            Ok(t) => t,
+            Err(_) => continue,
+        };
+
+        for threshold in &thresholds {
+            let (title, urgency) = match threshold {
+                100 => (
+                    format!("Budget exceeded: {}", cat_name),
+                    "critical",
+                ),
+                _ => (
+                    format!("Budget {}% used: {}", threshold, cat_name),
+                    "normal",
+                ),
+            };
+
+            let progress = db.get_budget_progress(budget.category_id, month)
+                .ok().flatten();
+            let body = if let Some((budget_amt, spent, pct)) = progress {
+                format!("{:.2} of {:.2} spent ({:.0}%)", spent, budget_amt, pct)
+            } else {
+                String::new()
+            };
+
+            send_notification(&title, &body, urgency);
+            let _ = db.record_notification(budget.category_id, month, *threshold);
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_send_notification_does_not_panic() {
+        // Should not panic even if notify-send is not installed
+        send_notification("Test", "Body", "normal");
+    }
+}