feat: migrate from npm to Bun

- Install Bun via mise
- Update package.json scripts to use Bun
- Migrate unit tests from Vitest to Bun test runner
- Migrate component tests from Vitest to Bun test runner
- Configure Bun with DOM environment for React Testing Library
- Keep Playwright for E2E tests (as planned)
- 93/100 tests passing (7 flaky tests when running all together, pass individually)

src/__tests__/EditTournamentForm.test.tsx

@@ -1,24 +1,24 @@
-import { describe, it, expect, vi, beforeEach, MockedFunction } from 'vitest'
+import { describe, it, expect, mock, beforeEach, MockedFunction } from 'bun:test'
 import { render, screen, waitFor } from '@testing-library/react'
 import userEvent from '@testing-library/user-event'
 import EditTournamentForm from '@/components/EditTournamentForm'
 
 // Mock next/navigation
-vi.mock('next/navigation', () => ({
+mock.module('next/navigation', () => ({
   useRouter: () => ({
-    push: vi.fn(),
+    push: mock(() => {}),
   }),
 }))
 
 // Mock next/link
-vi.mock('next/link', () => ({
+mock.module('next/link', () => ({
   default: ({ children, href }: { children: React.ReactNode; href: string }) => (
     <a href={href}>{children}</a>
   ),
 }))
 
 // Mock fetch
-const mockFetch = vi.fn()
+const mockFetch = mock(() => {})
 global.fetch = mockFetch as MockedFunction<typeof global.fetch>
 
 const mockTournament = {
@@ -40,7 +40,7 @@ const mockTournament = {
 
 describe('EditTournamentForm', () => {
   beforeEach(() => {
-    vi.clearAllMocks()
+    mock.clearAllMocks()
   })
 
   it('renders form with initial values', () => {