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/src/viewers/SchemaLinkingViewer.test.tsx

@@ -0,0 +1,57 @@
+import { render, screen, waitFor } from "@testing-library/react";
+import userEvent from "@testing-library/user-event";
+import { SchemaLinkingViewer } from "./SchemaLinkingViewer";
+
+vi.mock("./mermaid", () => ({
+  renderMermaid: vi.fn().mockResolvedValue('<svg data-testid="mm"></svg>'),
+}));
+
+const smallLinking = {
+  question: "quante ordini?",
+  candidates: [
+    { kind: "table" as const, name: "orders", decision: "promoted" as const },
+    { kind: "column" as const, name: "orders.id", decision: "promoted" as const },
+    { kind: "table" as const, name: "users", decision: "excluded" as const },
+  ],
+  joins: [{ from: "orders", to: "orders.id" }],
+  excluded: [{ kind: "table" as const, name: "users" }],
+  open_questions: [],
+};
+
+const bigLinking = {
+  question: "grossa query",
+  candidates: Array.from({ length: 46 }, (_, i) => ({
+    kind: "table" as const,
+    name: `table_${i}`,
+    decision: "promoted" as const,
+  })),
+  joins: [],
+  excluded: [],
+  open_questions: [],
+};
+
+test("(a) default renders the mermaid container (mocked svg appears)", async () => {
+  render(<SchemaLinkingViewer linking={smallLinking} />);
+  await waitFor(() => {
+    expect(screen.getByTestId("mm")).toBeInTheDocument();
+  });
+});
+
+test("(b) clicking toggle switches to table listing a promoted table name", async () => {
+  render(<SchemaLinkingViewer linking={smallLinking} />);
+  // wait for mermaid to load
+  await waitFor(() => expect(screen.getByTestId("mm")).toBeInTheDocument());
+
+  const toggle = screen.getByRole("button", { name: /tabella/i });
+  await userEvent.click(toggle);
+
+  expect(screen.getByText("orders")).toBeInTheDocument();
+  expect(screen.queryByTestId("mm")).not.toBeInTheDocument();
+});
+
+test("(c) oversized linking (>45) shows cap notice", () => {
+  render(<SchemaLinkingViewer linking={bigLinking} />);
+  expect(
+    screen.getByText(/troppi elementi per il grafico/i)
+  ).toBeInTheDocument();
+});