Notification panel needs a Clear all button

[tomazzi14]

The notification dropdown lists items with "Mark all read" but there's no way to clear the inbox without manually deleting rows in the DB.

Add a "Clear all" button next to "Mark all read" in the notification panel header that deletes all the user's notifications (uses the existing DELETE RLS policy).

Acceptance:

• Button visible only when items.length > 0
• Confirmation prompt before deletion (it's destructive)
• Optimistic UI: the list empties immediately, the DB delete runs in the background