feat(frontend): results panel (AGGrid + preview/export)

Add ResultsPanel component with AGGrid v36 for tabular preview results,
bold scalar display for 1×1 results, limit selector (10 / tutti), and
Esporta CSV button delegating export to the backend. MSW-tested with 3
passing scenarios (grid cells, scalar, export endpoint call).

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

frontend/package.json

@@ -14,6 +14,8 @@
     "@base-ui/react": "^1.6.0",
     "@fontsource-variable/geist": "^5.2.9",
     "@tanstack/react-query": "^5.59.0",
+    "ag-grid-community": "^36.0.0",
+    "ag-grid-react": "^36.0.0",
     "class-variance-authority": "^0.7.1",
     "clsx": "^2.1.1",
     "lucide-react": "^1.21.0",