feat(renderer): MermaidLazy (loads mermaid on first use, error UI)

src/renderer/components/preview/MermaidLazy.tsx

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+import { useEffect, useRef, useState } from 'react';
+
+interface Props {
+  code: string;
+}
+
+let mermaidModule: typeof import('mermaid').default | null = null;
+let initialized = false;
+
+async function getMermaid() {
+  if (!mermaidModule) {
+    const mod = await import('mermaid');
+    mermaidModule = mod.default;
+  }
+  if (!initialized) {
+    mermaidModule.initialize({ startOnLoad: false, theme: 'default' });
+    initialized = true;
+  }
+  return mermaidModule;
+}
+
+export function MermaidLazy({ code }: Props) {
+  const [svg, setSvg] = useState<string | null>(null);
+  const [error, setError] = useState<string | null>(null);
+  const idRef = useRef(`mermaid-${Math.random().toString(36).slice(2)}`);
+
+  useEffect(() => {
+    let cancelled = false;
+    getMermaid()
+      .then((m) => m.render(idRef.current, code))
+      .then((rendered) => {
+        if (!cancelled) setSvg(rendered);
+      })
+      .catch((err: Error) => {
+        if (!cancelled) setError(err.message);
+      });
+    return () => {
+      cancelled = true;
+    };
+  }, [code]);
+
+  if (error) return <div className="rounded border border-destructive/40 bg-destructive/5 p-2 text-xs text-destructive">{error}</div>;
+  if (!svg) return <div className="text-xs text-muted-foreground">Loading diagram…</div>;
+  return <div data-testid="mermaid-output" dangerouslySetInnerHTML={{ __html: svg }} />;
+}

tests/component/preview/MermaidLazy.test.tsx

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+import { describe, it, expect, vi } from 'vitest';
+import { render, screen, waitFor } from '@testing-library/react';
+import { MermaidLazy } from '@/components/preview/MermaidLazy';
+
+vi.mock('mermaid', () => ({
+  default: {
+    initialize: vi.fn(),
+    render: vi.fn().mockResolvedValue('<svg></svg>'),
+  },
+}));
+
+describe('MermaidLazy', () => {
+  it('renders an svg after mermaid resolves', async () => {
+    render(<MermaidLazy code="graph TD; A-->B" />);
+    await waitFor(() => {
+      expect(screen.getByTestId('mermaid-output').innerHTML).toContain('<svg');
+    });
+  });
+
+  it('shows an error message if mermaid throws', async () => {
+    const mermaid = (await import('mermaid')).default as any;
+    mermaid.render.mockRejectedValueOnce(new Error('mermaid failed'));
+    render(<MermaidLazy code="bad code" />);
+    await waitFor(() => {
+      expect(screen.getByText(/mermaid failed/i)).toBeInTheDocument();
+    });
+  });
+});