feat(tests): Jest backend (38 tests), Jest RTL frontend (28 tests), Playwright E2E spec

frontend/tests/Button.test.jsx

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+import React from 'react';
+import { render, screen, fireEvent } from '@testing-library/react';
+import Button from '../src/components/ui/Button';
+
+describe('Button component', () => {
+  it('renders with children text', () => {
+    render(<Button>Click me</Button>);
+    expect(screen.getByRole('button', { name: /click me/i })).toBeInTheDocument();
+  });
+
+  it('calls onClick when clicked', () => {
+    const onClick = jest.fn();
+    render(<Button onClick={onClick}>Click</Button>);
+    fireEvent.click(screen.getByRole('button'));
+    expect(onClick).toHaveBeenCalledTimes(1);
+  });
+
+  it('is disabled when disabled prop is true', () => {
+    render(<Button disabled>Submit</Button>);
+    expect(screen.getByRole('button')).toBeDisabled();
+  });
+
+  it('is disabled and shows spinner when loading', () => {
+    render(<Button loading>Submit</Button>);
+    const btn = screen.getByRole('button');
+    expect(btn).toBeDisabled();
+    expect(btn.querySelector('svg')).toBeTruthy();
+  });
+
+  it('renders danger variant', () => {
+    render(<Button variant="danger">Delete</Button>);
+    const btn = screen.getByRole('button');
+    expect(btn.className).toContain('bg-red-600');
+  });
+
+  it('does not call onClick when disabled', () => {
+    const onClick = jest.fn();
+    render(<Button disabled onClick={onClick}>Click</Button>);
+    fireEvent.click(screen.getByRole('button'));
+    expect(onClick).not.toHaveBeenCalled();
+  });
+});