Add desktop notifications with multi-tab handling

[NicoPennec]

Problem

• Users have to keep the Kitsu tab active to know about new events

Solution

• Native browser notification fired on each new event, opt-in via a banner on the Notifications page or a toggle in the user menu
  • Multi-tab handling: only one tab pops the notification
  • No duplicate notification when the Notifications page is already in the foreground
  • NDA-safe: only the title and entity name, never the comment or reply text