feat(frontend): schema-linking viewer (mermaid+table) + markdown view

TDD: SchemaLinkingViewer with mocked renderMermaid, toggle flowchart↔table,
≤45 element cap with Italian notice. MarkdownView renders mermaid fences.

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

frontend/package.json

@@ -17,9 +17,12 @@
     "class-variance-authority": "^0.7.1",
     "clsx": "^2.1.1",
     "lucide-react": "^1.21.0",
+    "mermaid": "^11.16.0",
     "next-themes": "^0.4.6",
     "react": "^18.3.1",
     "react-dom": "^18.3.1",
+    "react-markdown": "^10.1.0",
+    "remark-gfm": "^4.0.1",
     "sonner": "^2.0.7",
     "tailwind-merge": "^3.6.0",
     "tw-animate-css": "^1.4.0",