[dajiaohuang]

Summary

Correct the layout default in the Posting 2 configuration reference from horizontal to vertical.

Settings.layout defaults to vertical. Starting Posting 2 with a fresh isolated configuration also produces the vertical layout (request above response), so the current reference describes a different default from what a new user gets. This changes only the documented default, not application behavior.

Targets 2.x as requested by the current contribution guide for Python maintenance.

Validation

• Checked the reference against an actual fresh-config Posting startup with Textual Pilot: the documented-default comparison failed before this change and passed afterward.
• mkdocs build --strict passed using the locked documentation dependencies.
• git diff --check passed.
• No application code changed; the full Python test suite was not run.