Support Sidebery vertical tabs

### Is your feature request related to a problem?

At minimum - it should be supported by `userChrome.hidden.sidebar_header.vertical_tab_only` setting.

### The solution I'd like

At best - the extension appearance should match the rest of the browser. As I noticed, there are some styles specific to Tree Style Tab and similar extensions already present in `leptonChrome.css`

### Screenshots or video

_No response_

### Alternatives I've considered

_No response_

### Additional context

_No response_

Start by reviewing the `userChrome.hidden.sidebar_header.vertical_tab_only` setting and the existing Sidebery- or Tree Style Tab-related styles in `leptonChrome.css`. Compare Sidebery's vertical-tab appearance with the browser UI and determine the minimum styling needed for setting support. Done means the setting works and Sidebery's appearance is consistent with the rest of the browser.

Written by the indexing model from the issue text.