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/commands/notifications.rs

@@ -0,0 +1,8 @@
+// Notification commands — desktop notification via notify-rust
+
+use crate::notifications;
+
+#[tauri::command]
+pub fn notify_desktop(title: String, body: String, urgency: String) -> Result<(), String> {
+    notifications::send_desktop_notification(&title, &body, &urgency)
+}