feat: disable desktop notifications by default

Changed NotificationManager::new() to set enabled: false
This completely disables all desktop notifications in the app.

Modified:
- src/notifications.rs:32 - enabled: true -> false

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/notifications.rs

@@ -29,7 +29,7 @@ impl NotificationManager {
     /// Creates a new notification manager with default settings
     pub fn new() -> Self {
         Self {
-            enabled: true,
+            enabled: false,
             muted_chats: HashSet::new(),
             only_mentions: false,
             show_preview: true,