Fix: correct Playwright Docker tag, sequential Lighthouse audits, devtools throttling

- Pin playwright npm to 1.59.1 to match Docker image tag
- Update base image to mcr.microsoft.com/playwright:v1.59.1-jammy
- Run Lighthouse desktop and mobile sequentially (parallel caused port conflict)
- Switch to devtools throttling (simulate caused LanternError on localhost)
- Skip browser download during npm ci (image has browsers pre-installed)

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

package.json

@@ -11,7 +11,7 @@
   "dependencies": {
     "chrome-launcher": "^1.1.2",
     "lighthouse": "^12.2.1",
-    "playwright": "^1.44.0"
+    "playwright": "1.59.1"
   },
   "engines": {
     "node": ">=20"