[idea] Herdr app: turn off the frame on a lone pane without leaving the shipped layout

[chatoo2412]

idea / problem

In the Herdr app, whether a tab's only pane gets the pane frame (border, background, and header) is set by pane.lone in layouts/<name>.json. The shipped main layout sets "lone": true.

Shipped layouts are read-only: they are replaced on every start, and a manual edit is moved to main-edited while Herdr's version is restored. So the only way to turn the frame off for a lone pane is to clone main, and the clone stops receiving later updates to main (structure and mods).

settings.json already covers per-person look (fonts, sizes, theme, and pane_scrollbars), and the docs say the pane header height and border/focus look are saved as clicks in layouts/.state/. lone is in neither.

Seen on Herdr app 0.1.13, and still the case on master (2405bf3): lone is read only from the layout's pane frame.

requested change

Make lone a per-person override, either as a settings.json key or as a saved click like the border/focus look, so a shipped layout can be kept while a lone pane is shown without a frame.

why you want this

Whether a lone pane has a frame is a matter of personal look, like the other settings.json keys. Today changing it costs every later update to main, because the only way is a clone that has to be kept in sync by hand.

Related: herdrdev/herdr-app#456, another case where a per-person change means editing the shipped layout. First filed as #5847 and moved here as suggested there.