Add mark all as read for notifications

Make it easy to clear notification backlog in one action.
Reduce the friction and stress caused by a large unread notification count without hiding notification state entirely.

The current notifications experience can feel overwhelming when the unread count grows large.
Hiding the badge would reduce visual noise, but the simpler and more direct improvement is to let users clear notifications in bulk.
Today, notifications can be marked as read one at a time, but there is no bulk Mark all as read action.
The current sidebar badge is shown whenever the unread count is greater than zero.
Existing screenshot of the sidebar badge:

The sidebar notifications entry point lives in src/components/navbar/NavNotifications.tsx, where the unread badge is rendered from unreadCount.
The notifications list UI lives in src/components/dashboard/RecentMentions/MentionsList.tsx.
Current behavior in MentionsList.tsx supports per-notification read actions only:
an explicit mark-as-read control for individual unread items
an implicit mark-as-read action when opening an individual notification
There is currently no bulk action in the notifications UI, no select-all flow, and no call path for a bulk mark-all-read action.
The requirement is to add a clear bulk Mark all as read action in the notifications experience.
Add Mark all as read to the notifications popover next to View All Notifications.
Keep the existing single-notification mark as read behavior available alongside the new bulk action.
Single-notification mark as read should run without a confirmation step.
Mark all as read should require a confirmation step before applying the action.
After a successful bulk action, the UI should immediately clear the sidebar badge, refresh the notifications list, and empty the unread view.
This behavior is intended for every user who has notifications.

Attik