Feature summary:
Add a confirmation step, when a user clicks any of the 3 types of "Mark all as read" buttons.

Use case(s):

• As a user who has many notifications, especially ones from other wikis, I sometimes accidentally click one of the "mark as read" or "mark all as read" buttons. This step is impossible to undo, and it is often hard to track down where the Notifications came from (because local notifications which are read are only stored on that wiki). I want a confirmation step, to prevent these accidents.

Benefits:

• Reduce the quantity of missed-notifications because they were accidentally marked as "read".

Screenshots
These are the 3 types of "Mark all as read" buttons that can lead to missed notifications. The 1st and 3rd (which both cover cross-wiki Notifications) are the most crucial to have a confirmation step for.

Related links:

• https://meta.wikimedia.org/wiki/Community_Wishlist/W472 ("Confirmation dialog to prevent accidental clicks on Mark All As Read for notifications")
• https://en.wikipedia.org/wiki/Wikipedia:Village_pump_(technical)#Accidental_click_on_multi-notification's_'mark_as_read'

Open questions:

• Whether a confirmation step is needed for local-only bundled Notifications.