Add collapsible Kanban columns

[eysenfalk]

Summary

• add an accessible collapse control to flat Kanban column headers
• persist collapsed column keys in the Bases view's ephemeral state
• restore the state when the view is recreated and ignore malformed saved state
• keep swimlane headers unchanged

Behavior

Collapse affects only the selected column. The column stays visible as a narrow header, so it can be expanded again. The state belongs to the current Bases view rather than plugin-wide settings.

Validation

• TZ=UTC npx jest tests/unit/issues/issue-1341-kanban-column-collapse-state.test.ts --runInBand
• npm test
• npm run typecheck
• npm run lint
• npm run build:test
• isolated Obsidian runtime: toggle, sibling isolation, persisted restore after view recreation, and no runtime errors

Runtime preview

The isolated runtime check covered expanded and collapsed columns plus restoration after recreating the active Bases view. Screenshot evidence was captured during validation but is not committed to the plugin repository.

Scope

This PR contains only column collapse and persistence. It does not include status colors or drag-animation changes.