Keyboard shortcuts on notifications column

I *love* Phanpy having only just discovered it. Thank you!

My only niggle is that I *love* using the keyboard shortcuts, but I can't use them on the notifications column, which has no focus. I run a multi-column setup so I can just press a number to focus a column, then j/k to navigate around, then r to reply, but I can't do that on the notifications column. I wondered if it was due to mixed content, but I can't do it even when it's set to "Only mentions".

**Describe the solution you'd like**
I'd love for the notifications column to have the same keyboard focus as all the other columns so I can quickly reply to mentions using the keyboard. It would be fine if only mentions were able to have keyboard focus, although perhaps that would cause more problems, say if the first mention was off the bottom of the screen. Ideally I'd just like to do everything on the keyboard :)

Start by running Phanpy in a multi-column setup and tracing the keyboard shortcut handling for column focus, j/k navigation, and reply actions. Compare the notifications column with other columns, including the “Only mentions” mode; done means notifications can be focused and navigated with the keyboard and mentions can be replied to consistently.

Written by the indexing model from the issue text.