Vertical tabs, resizable and multi-column

The side tab bar can be dragged to a width that is remembered, and flows into multiple columns once there is room for a whole one. Tab titles in this fork are paths and session names, which a horizontal strip truncates to nothing, so vertical is the default here.

Tab titles in this fork are working directories and session names. A horizontal strip truncates them well before the useful part, and Tabby's side tab bar was a fixed width with a single column.

The side bar can be dragged to a width that is persisted, and flows into multiple columns as it gets wider — a column narrower than the minimum is not created, so columns appear only once there is room for a whole one. Vertical is the default here.

• appearance.tabsLocation: 'left'

  Was top. Only affects profiles with no value saved.

• appearance.sideTabBarWidth: 200

  Persisted when the splitter is dragged.

• appearance.sideTabBarMultiColumn: false

  Let the bar flow into multiple columns.

• appearance.sideTabBarMinColumnWidth: 200

  A column narrower than this is not created.

• appearance.colorSchemeMode: 'auto'

  Was dark, which ignored the OS setting on a fresh profile.

• Every changed default is a line that conflicts on rebase, so they are kept to a minimum and listed in CLAUDE.md. colorSchemeMode: auto is a changed default — following the OS scheme is upstream Tabby's own mechanism, and auto was already a supported value.
• Separate light and dark colour schemes are stock upstream Tabby. Nothing here added that.