feat(ui): improve inventory navigation

[terrytan95]

Description

Makes inventory health filters readable at a glance and allows skill folders to collapse in the file tree. The implementation keeps the existing inventory structure while adding native button semantics and compact folder transitions.

Key Changes

• Label the All, Healthy, and Needs Review inventory filters.
• Compact the filter rail to preserve workspace width.
• Toggle file-tree folders with expanded-state accessibility.
• Reset folder state when switching skills.

Verification & Testing

• yarn typecheck
• Environment-specific verification: visual QA artifacts remain uncommitted.

Impact & Risk

None.