[web] Make "Mark as unread" button easier to click reliably

Summary:
Context: https://linear.app/comm/issue/ENG-992/mark-as-unread-button-can-be-hard-to-press-on-web

Users reported that sometimes the "Mark as unread" button would disappear before they were able to click it. It looks like this was because there was a 1px gap (see image below) between the menu button (vertical ellipsis) and the toggle unread status button ("Mark as unread") where onMouseLeave would trigger and hide the toggle unread status button.

I was able to reproduce this issue consistently when moving the cursor slowly from the vertical ellipsis to "Mark as unread" button, but was able to avoid the issue by moving my cursor "fast." Not sure why onMouseLeave doesn't trigger if you move the cursor back within the div fast enough, but that's the behavior I observed.

This diff basically closes that 1px gap and gets things working reliably.

Test Plan: Was able to consistently mark a thread as read/unread without the "Mark as unread" button disappearing.

Reviewers: tomek, abosh, varun, jacek, ashoat

Reviewed By: abosh, varun, ashoat

Subscribers: ashoat, adrian

Differential Revision: https://phab.comm.dev/D4669