feat(renderer): add UpdateBanner for non-blocking update notifications

src/renderer/components/UpdateBanner.tsx

@@ -0,0 +1,44 @@
+import { useUpdaterStore } from '@/lib/updater-store';
+import { ipc } from '@/lib/ipc';
+import { toast } from 'sonner';
+
+export function UpdateBanner() {
+  const { state, version, percent, install, check } = useUpdaterStore();
+
+  if (state === 'idle' || state === 'checking') return null;
+
+  if (state === 'error') {
+    return (
+      <div data-testid="update-banner" role="status" className="bg-amber-50 border-b border-amber-200 px-4 py-2 text-sm">
+        Couldn't check for updates.{' '}
+        <button onClick={async () => { try { await check(); } catch (e: any) { toast.error(e.message); } }} className="underline">
+          Try again
+        </button>
+      </div>
+    );
+  }
+
+  if (state === 'downloading') {
+    return (
+      <div data-testid="update-banner" className="bg-blue-50 border-b border-blue-200 px-4 py-2 text-sm">
+        Downloading update… {Math.round(percent)}%
+      </div>
+    );
+  }
+
+  if (state === 'ready') {
+    return (
+      <div data-testid="update-banner" className="bg-green-50 border-b border-green-200 px-4 py-2 text-sm flex items-center gap-3">
+        <span>v{version} is ready.</span>
+        <button onClick={() => ipc.app.openExternal(`https://github.com/amitwh/markdown-converter/releases/tag/v${version}`)} className="underline">
+          View release notes
+        </button>
+        <button onClick={install} className="px-3 py-1 rounded bg-brand text-white">
+          Restart to update
+        </button>
+      </div>
+    );
+  }
+
+  return null;
+}

tests/component/UpdateBanner.test.tsx

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+import { describe, expect, test, beforeEach, vi } from 'vitest';
+import { render, screen, fireEvent } from '@testing-library/react';
+import { useUpdaterStore } from '@/lib/updater-store';
+import { UpdateBanner } from '@/components/UpdateBanner';
+
+vi.mock('sonner', () => ({ toast: { success: vi.fn(), error: vi.fn() } }));
+vi.mock('@/lib/ipc', () => ({
+  ipc: {
+    app: { openExternal: vi.fn() },
+    updater: { check: vi.fn(), install: vi.fn() },
+  },
+}));
+
+describe('UpdateBanner', () => {
+  beforeEach(() => {
+    useUpdaterStore.setState({ state: 'idle', version: null, percent: 0 });
+  });
+
+  test('renders nothing when state is idle', () => {
+    const { container } = render(<UpdateBanner />);
+    expect(container.firstChild).toBeNull();
+  });
+
+  test('shows version and restart button when state is ready', () => {
+    useUpdaterStore.setState({ state: 'ready', version: '5.0.2' });
+    render(<UpdateBanner />);
+    expect(screen.getByText(/5\.0\.2/)).toBeInTheDocument();
+    expect(screen.getByRole('button', { name: /restart/i })).toBeInTheDocument();
+  });
+
+  test('shows download progress when state is downloading', () => {
+    useUpdaterStore.setState({ state: 'downloading', percent: 42 });
+    render(<UpdateBanner />);
+    expect(screen.getByText(/42%/)).toBeInTheDocument();
+  });
+
+  test('shows error copy when state is error', () => {
+    useUpdaterStore.setState({ state: 'error' });
+    render(<UpdateBanner />);
+    expect(screen.getByText(/couldn.?t check/i)).toBeInTheDocument();
+  });
+
+  test('restart button calls ipc.updater.install', () => {
+    useUpdaterStore.setState({ state: 'ready', version: '5.0.2' });
+    render(<UpdateBanner />);
+    fireEvent.click(screen.getByRole('button', { name: /restart/i }));
+    expect(useUpdaterStore.getState().install).toBeDefined();
+  });
+});