Notify on internal notes only when @mentioned

Today the notification setting for internal notes is all-or-nothing: any internal note in any chat triggers a sound/push, even when nobody is tagged. In a team with several agents this means constant noise, so most people end up disabling internal-note notifications entirely, and then miss the notes where they are tagged.

Request: add a third option for internal notes: "Only when I'm @mentioned" (alongside the current on/off). Tagging someone in a note already creates a task for them, so the intent is explicit; the notification should follow that same logic.

Nice to have: same option for group chats (notify only on @mention), which is how WhatsApp itself behaves.