test: add Playwright smoke suite (cookie banner, lang switcher, FamilyAtlas, WritingIssue)

34 tests across desktop + mobile (Chromium/Pixel 5). Verifies all client-side
React islands and interactive features that astro build/WebFetch cannot confirm.
API-dependent WritingIssue shows graceful error against local preview; resolves
on the live site where the PHP backend is available.

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

playwright.config.ts

@@ -0,0 +1,17 @@
+import { defineConfig, devices } from "@playwright/test";
+
+export default defineConfig({
+  testDir: "./tests",
+  timeout: 15_000,
+  use: { baseURL: "http://localhost:4321" },
+  webServer: {
+    command: "npm run preview",
+    url: "http://localhost:4321",
+    reuseExistingServer: true,
+    timeout: 30_000,
+  },
+  projects: [
+    { name: "desktop", use: { ...devices["Desktop Chrome"] } },
+    { name: "mobile", use: { ...devices["Pixel 5"] } },
+  ],
+});