page.PrimaryColumnIndex isn't configurable

### Description

I saw code in internal/glance/glance.go "page.PrimaryColumnIndex = -1", and internal/glance/config.go "PrimaryColumnIndex int8 `yaml:"-"`". It can't be set by yaml, always be -1. So the default column on mobile is the full size one. I hope I can set in yaml, the default column shown on mobile.

Start with internal/glance/glance.go and internal/glance/config.go, where PrimaryColumnIndex is assigned -1 and excluded from YAML. Make the setting configurable through YAML while preserving the existing default, then verify that the configured value determines the default column shown on mobile.

Written by the indexing model from the issue text.