feat(frontend): SQL/CTE viewer with shiki highlighting

TDD: SqlViewer renders collapsible CTE blocks with name/field-count/
test-status badges, shiki-highlighted SQL body (dangerouslySetInnerHTML),
per-field comments, and a vertical/horizontal layout toggle.
highlight.ts wraps shiki as a lazy singleton; tests mock it for
determinism (pattern mirrors mermaid.ts).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/package.json

@@ -23,6 +23,7 @@
     "react-dom": "^18.3.1",
     "react-markdown": "^10.1.0",
     "remark-gfm": "^4.0.1",
+    "shiki": "^4.3.0",
     "sonner": "^2.0.7",
     "tailwind-merge": "^3.6.0",
     "tw-animate-css": "^1.4.0",