[codex] optimize web CI reporter-time stability

[yangshu2087]

Summary

• Reduce Playwright context/page churn by keeping /app open for grouped generation scenarios and reusing the same page/session per group test.
• Split the previous slow mixed generation case into two grouped tests (tweet/thread, article/diagram) and add per-scenario timing logs to isolate hotspots.
• Add a CI step-duration table in Actions summary to track cache restore/save, install Chromium, typecheck/test/build wall time for long-term trend analysis.

Test Plan

• git diff --check
• ruby -e "require 'yaml'; YAML.load_file('.github/workflows/ci.yml'); puts 'workflow yaml ok'"
• npm_config_cache=/tmp/draftorbit-npm-cache npx pnpm@10.23.0 --filter @draftorbit/web typecheck
• NEXT_PUBLIC_API_URL=/__api NEXT_PUBLIC_ENABLE_LOCAL_LOGIN=true CI=true WEB_PLAYWRIGHT_PORT=3320 WEB_PLAYWRIGHT_WORKERS=2 WEB_PLAYWRIGHT_REPORTER_TARGET_SECONDS=10 WEB_PLAYWRIGHT_REPORTER_HARD_BUDGET_SECONDS=12 WEB_PLAYWRIGHT_ENFORCE_BUDGET=1 GITHUB_STEP_SUMMARY=/tmp/draftorbit-web-perf-opt-summary.md npm_config_cache=/tmp/draftorbit-npm-cache npx pnpm@10.23.0 --filter @draftorbit/web test
• NEXT_PUBLIC_API_URL=http://127.0.0.1:4311 npm_config_cache=/tmp/draftorbit-npm-cache npx pnpm@10.23.0 --filter @draftorbit/web build
• GitHub Actions Web required checks on PR