feat: add testing infrastructure — Vitest, Playwright, CI, git hooks

Add comprehensive testing covering 420 unit/handler tests across 24 Vitest
files, 9 Playwright E2E specs, accessibility scans, and visual regression.
Includes GitHub Actions CI, Husky pre-push hook, and TESTING.md docs.

playwright.config.js

@@ -0,0 +1,31 @@
+import { defineConfig } from '@playwright/test'
+
+export default defineConfig({
+  testDir: './e2e',
+  outputDir: 'e2e/test-results',
+  snapshotDir: 'e2e/__screenshots__',
+  fullyParallel: true,
+  forbidOnly: !!process.env.CI,
+  retries: process.env.CI ? 1 : 0,
+  workers: process.env.CI ? 1 : undefined,
+  reporter: 'html',
+  use: {
+    baseURL: 'http://localhost:3000',
+    trace: 'on-first-retry',
+  },
+  projects: [
+    {
+      name: 'chromium',
+      use: { browserName: 'chromium' },
+    },
+  ],
+  webServer: {
+    command: 'npm run dev',
+    url: 'http://localhost:3000',
+    reuseExistingServer: !process.env.CI,
+    env: {
+      NUXT_PUBLIC_COMING_SOON: 'false',
+      NODE_ENV: 'development',
+    },
+  },
+})