Add an `o` key that opens an inline prompt to set the download folder,
shown in the `?` cheatsheet and the sidebar footer. Reuses Panel +
TextField and the captureMode/effective-region mechanism the help
overlay already uses, so section keymaps stay quiet while typing a path.
No new Store field (editingFolder is local App state, like showHelp).
expandHome resolves a leading ~/ and ~\ so a path pasted from another
OS still works; the folder is created with fs.mkdir(recursive) and the
change fails soft, keeping the old folder if it cannot be created. The
setting only feeds new downloads (queue.add reads it at add-time), so
in-progress downloads and existing seeds keep their own dir and are
never interrupted or relocated.
