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>

Dockerfile

@@ -1,10 +1,10 @@
-FROM mcr.microsoft.com/playwright/node:20-noble
+FROM mcr.microsoft.com/playwright:v1.59.1-jammy
 
 WORKDIR /app
 
 # Install dependencies first (layer cache)
 COPY package*.json ./
-RUN npm ci --omit=dev
+RUN PLAYWRIGHT_SKIP_BROWSER_DOWNLOAD=1 npm ci --omit=dev
 
 # Copy source
 COPY src/ ./src/