Add unit testing infrastructure with Vitest

Set up Vitest for both server (Node + Supertest) and client (jsdom + React
Testing Library). Extract Express app into app.js for testability. Add example
tests covering bills validation, bills route CRUD, ThemeContext, and App nav
rendering. Update CLAUDE.md with testing docs and requirement to write tests
with features.

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

client/src/__tests__/App.test.jsx

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+import { describe, it, expect } from 'vitest';
+import { render, screen } from '@testing-library/react';
+import { MemoryRouter } from 'react-router-dom';
+import { ThemeProvider } from '../ThemeContext.jsx';
+import App from '../App.jsx';
+
+function renderApp(initialRoute = '/') {
+  return render(
+    <MemoryRouter initialEntries={[initialRoute]}>
+      <ThemeProvider>
+        <App />
+      </ThemeProvider>
+    </MemoryRouter>
+  );
+}
+
+describe('App', () => {
+  it('renders the navigation bar', () => {
+    renderApp();
+
+    expect(screen.getByText('Budget')).toBeInTheDocument();
+    expect(screen.getByText('Paychecks')).toBeInTheDocument();
+    expect(screen.getByText('Bills')).toBeInTheDocument();
+    expect(screen.getByText('Financing')).toBeInTheDocument();
+    expect(screen.getByText('Settings')).toBeInTheDocument();
+  });
+
+  it('renders theme toggle button', () => {
+    renderApp();
+
+    expect(screen.getByRole('button', { name: /toggle theme/i })).toBeInTheDocument();
+  });
+});

client/src/__tests__/ThemeContext.test.jsx

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+import { describe, it, expect, beforeEach } from 'vitest';
+import { render, screen } from '@testing-library/react';
+import userEvent from '@testing-library/user-event';
+import { ThemeProvider, useTheme } from '../ThemeContext.jsx';
+
+function ThemeDisplay() {
+  const { theme, toggle } = useTheme();
+  return (
+    <div>
+      <span data-testid="theme">{theme}</span>
+      <button onClick={toggle}>Toggle</button>
+    </div>
+  );
+}
+
+describe('ThemeContext', () => {
+  beforeEach(() => {
+    localStorage.clear();
+    document.documentElement.removeAttribute('data-theme');
+  });
+
+  it('defaults to light theme when no preference', () => {
+    // jsdom defaults to no prefers-color-scheme match
+    render(
+      <ThemeProvider>
+        <ThemeDisplay />
+      </ThemeProvider>
+    );
+
+    expect(screen.getByTestId('theme').textContent).toBe('light');
+  });
+
+  it('reads stored theme from localStorage', () => {
+    localStorage.setItem('theme', 'dark');
+
+    render(
+      <ThemeProvider>
+        <ThemeDisplay />
+      </ThemeProvider>
+    );
+
+    expect(screen.getByTestId('theme').textContent).toBe('dark');
+  });
+
+  it('toggles theme on button click', async () => {
+    const user = userEvent.setup();
+
+    render(
+      <ThemeProvider>
+        <ThemeDisplay />
+      </ThemeProvider>
+    );
+
+    expect(screen.getByTestId('theme').textContent).toBe('light');
+
+    await user.click(screen.getByText('Toggle'));
+
+    expect(screen.getByTestId('theme').textContent).toBe('dark');
+    expect(localStorage.getItem('theme')).toBe('dark');
+  });
+
+  it('sets data-theme attribute on document', () => {
+    localStorage.setItem('theme', 'dark');
+
+    render(
+      <ThemeProvider>
+        <ThemeDisplay />
+      </ThemeProvider>
+    );
+
+    expect(document.documentElement.getAttribute('data-theme')).toBe('dark');
+  });
+});

client/src/test/setup.js

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+import '@testing-library/jest-dom';
+
+// jsdom doesn't implement window.matchMedia
+Object.defineProperty(window, 'matchMedia', {
+  writable: true,
+  value: (query) => ({
+    matches: false,
+    media: query,
+    onchange: null,
+    addListener: () => {},
+    removeListener: () => {},
+    addEventListener: () => {},
+    removeEventListener: () => {},
+    dispatchEvent: () => {},
+  }),
+});