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/playwright.config.ts

@@ -0,0 +1,31 @@
+import { defineConfig, devices } from '@playwright/test'
+
+export default defineConfig({
+  testDir: '.',
+  fullyParallel: true,
+  forbidOnly: !!process.env.CI,
+  retries: process.env.CI ? 2 : 0,
+  workers: process.env.CI ? 1 : undefined,
+  reporter: 'html',
+  timeout: 30_000,
+
+  use: {
+    baseURL: 'http://localhost:5173',
+    trace: 'on-first-retry',
+  },
+
+  projects: [
+    {
+      name: 'chromium',
+      use: { ...devices['Desktop Chrome'] },
+    },
+  ],
+
+  webServer: {
+    command: 'pnpm dev',
+    url: 'http://localhost:5173',
+    reuseExistingServer: !process.env.CI,
+    timeout: 30_000,
+    cwd: '..',
+  },
+})