fix: When the terminal is insufficient to display all the commands, the cmdbar_bg configuration color does not fully take effect

[wugeer]

This Pull Request fixes/closes #2347.

It changes the following:

• Remove the line field from the Command struct, along with its corresponding initialization
• Remove the cmdbar_extra_lines_bg field from the Theme struct, along with its corresponding initialization

I followed the checklist:

• [ ] I added unittests
• [x] I ran make check without errors
• [x] I tested the overall application
• [x] I added an appropriate item to the changelog