Just like we have unread counters for reports and messages, add an overdue tasks counter in a bubble in the nav bar.

Tasks seems like a useful place for the bubble too, with the caveat that we only include "Overdue" and due "Today" tasks in the bubble. Dianna: "I want the tasks dot to tell me what things I need to do today, regardless of whether they are late or on-time. But not things that are due tomorrow." If I have three tasks in my list, one was due yesterday, one is due today, and one is due in two days, we'd like the bubble number to be "2". If I have no overdue tasks or tasks due today (only future tasks in my list) then the bubble should not appear, as there is nothing requiring my attention today.