fix: disable notifications in config defaults

The previous commit only changed NotificationManager::new() but the config
layer was overriding it with default_notifications_enabled() = true.

Changed default_notifications_enabled() to return false, which is the
authoritative source for notification settings.

Modified:
- src/config/mod.rs - default_notifications_enabled: true -> false

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

src/config/mod.rs

@@ -126,7 +126,7 @@ fn default_reaction_other_color() -> String {
 }
 
 fn default_notifications_enabled() -> bool {
-    true
+    false
 }
 
 fn default_show_preview() -> bool {