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/mermaid.ts

@@ -0,0 +1,12 @@
+let _initialized = false;
+
+export async function renderMermaid(def: string): Promise<string> {
+  const mermaid = (await import("mermaid")).default;
+  if (!_initialized) {
+    mermaid.initialize({ startOnLoad: false, theme: "default" });
+    _initialized = true;
+  }
+  const id = `mermaid-${Math.random().toString(36).slice(2)}`;
+  const { svg } = await mermaid.render(id, def);
+  return svg;
+}