test: add Playwright E2E infrastructure with smoke test

Set up Playwright with Chromium, dev server auto-start, test helpers
for seeding bots and programmatic auth. Added smoke spec that verifies
homepage and leaderboard load without crashes.

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

e2e/smoke.spec.ts

@@ -0,0 +1,19 @@
+import { test, expect } from '@playwright/test'
+
+test('homepage loads', async ({ page }) => {
+  await page.goto('/')
+  // Page should load without errors
+  await expect(page).toHaveTitle(/botfights/i)
+})
+
+test('leaderboard page loads', async ({ page }) => {
+  await page.goto('/leaderboard')
+  // Should render without console errors
+  const errors: string[] = []
+  page.on('console', msg => {
+    if (msg.type() === 'error') errors.push(msg.text())
+  })
+  await page.waitForTimeout(1000)
+  // Allow some errors (e.g., missing API data) but no crashes
+  expect(errors.filter(e => e.includes('TypeError') || e.includes('ReferenceError'))).toHaveLength(0)
+})