Add Playwright e2e suite for the landing page

- playwright.config.ts: desktop (1440) + mobile (Pixel 5) projects, static
  webServer over the exported out/; a PW_CHROME env override for the browser
- tests/e2e: render + all core sections, tracked download hrefs with no R2
  exposure, the live counter (mocked function), theme toggle, image loading,
  zero console errors, and responsive nav (desktop links / mobile menu reveal)
- navbar: hide the closed mobile menu with `invisible` so its links stay out of
  the tab order and the accessibility tree, not merely clipped
- add the test:e2e script and ignore test-results/ + playwright-report/

landing/package.json

@@ -7,7 +7,8 @@
     "dev": "next dev",
     "build": "next build",
     "start": "next start",
-    "preview": "wrangler pages dev out"
+    "preview": "wrangler pages dev out",
+    "test:e2e": "next build && playwright test"
   },
   "dependencies": {
     "next": "16.2.6",