Navigating Threads Panel: Back button

### Your use case

#### What would you like to do?
On the right panel, allow the (mouse) back button to return to previous panel .
#### Why would you like to do it?
To improve ease of use and make navigation more efficient.
#### How would you like to achieve it?
Allow mouse Back button navigate to the previously opened panel.

### Have you considered any alternatives?

1. Open Thread in a separate window/overlay e.g. The way Settings open in it's own window.

2. Add “Next unread” / “Previous unread” controls in the footer of the panel.

3. Add a back button to the footer of the panel.
- Similar to the jump-to-last-unread-message bubble except it's a back arrow.

### Additional context

Navigating Threads involve repeatedly moving the cursor up and down the panel.

When viewing a Thread, returning to the main Threads panel requires moving the cursor to the **top of the panel** (to click the back button).
Since the most recent Thread appears at the **bottom of the panel** users would have to move their cursor between these positions which seem inefficient for reading multiple threads.