Add response size and timing metrics to network stats

[christoph-jerolimov]

Summary

• Collect per-response body size and timing data during load tests via Playwright's request.sizes() and request.timing() APIs
• Add responseSizeSumKb, responseSizeMaxKb, responseTimeSumSec, and responseTimeAvgSec to the stats object
• Aggregate network stats (sum/avg) across all runs in the analyse script
• Make logStatsAndErrors async to support awaiting size and timing data

Test plan

• Run a load test locally and verify the new metrics appear in the Stats output
• Run analyse.mts against a JSON report and confirm the Network Stats table renders correctly

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