feat: change heading based on filter

[elle-l-em]

Description

Change the heading text so that it reflects the current tab. Previously, all tabs said "remaining", now they say:

• All: "N tasks"
• Active: "N tasks remaining" (same as before)
• Completed: "N tasks completed"

Motivation

I started using this and was confused that when I went to my completed tasks, it said those tasks were still "remaining". All the tab filters say "remaining" but I think of "remaining" tasks as the tasks still left to do, not the completed ones. Now the header text appropriately reflects the active filter.

Additional details

None

Related issues and pull requests

None

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[bsmth]

I'm going to close this due to inactivity, if you'd like to come back to this, please reply or reopen it! Thanks :)