[OPER-QA-8] Test Library canvas — browse, filter, and edit TestCase rows

feature_id: FEAT-studio-qa

## Context

Second canvas of the QA Tests tab. Renders TestCase rows in a filterable table (by feature, kind, status, assignee). Supports inline status edit (DRAFT -> ACTIVE), tag editing, and single-case detail drawer.

## Acceptance Criteria

- Route `/studio/[team]/qa/cases` renders the Test Library
- Component `TestLibrary` supports filters: feature (from feature-map), kind (TestKind enum), status (TestCaseStatus enum), assignee
- Row click opens detail drawer with all TestCase fields + linked TestRuns
- Status edit inline via dropdown; writes via `PATCH /api/studio/[team]/qa/cases/[id]`
- Empty state prompts running OPER-QA-1 autogen
- Vitest coverage: filter combinations, edit path, empty state
- E2E: filter by kind, edit status, verify persistence