Build tag sidebar UI (kind/tag tree with multi-select state) #12
Description
Build a tag sidebar that presents kinds and tags as a filterable tree with clear multi-select interaction.
Scope
- Render sidebar grouped by tag kind.
- Show tag-level counts from existing tag count queries.
- Support multi-select behavior and selected-state affordances.
- Keep sidebar state synchronized with URL-backed filter model.
Acceptance Criteria
- Kind/tag hierarchy is visible and navigable.
- Selecting/deselecting tags updates filter state and URL.
- Selected state is visually clear and keyboard accessible.
- Counts render correctly for current filter context.