feat: add OS + in-app notification system

notify-rust for desktop notifications, NotificationCenter.svelte with
bell icon, unread badge, history (max 100), 6 notification types.
Extended notification store with history and type support.

v2/src-tauri/src/notifications.rs

@@ -0,0 +1,31 @@
+// Desktop notification support via notify-rust
+
+use notify_rust::{Notification, Urgency};
+
+/// Send an OS desktop notification.
+/// Fails gracefully if the notification daemon is unavailable.
+pub fn send_desktop_notification(
+    title: &str,
+    body: &str,
+    urgency: &str,
+) -> Result<(), String> {
+    let urgency_level = match urgency {
+        "critical" => Urgency::Critical,
+        "low" => Urgency::Low,
+        _ => Urgency::Normal,
+    };
+
+    match Notification::new()
+        .summary(title)
+        .body(body)
+        .appname("BTerminal")
+        .urgency(urgency_level)
+        .show()
+    {
+        Ok(_) => Ok(()),
+        Err(e) => {
+            tracing::warn!("Desktop notification failed (daemon unavailable?): {e}");
+            Ok(()) // Graceful — don't propagate to frontend
+        }
+    }
+}