Run browser tests in parallel shards with 4 workers

[MrAlders0n]

The browser job ran 666 tests (111 tests × 6 browser projects) on one runner with 2 workers, about 17 minutes of wall time.

• Split the browser job into one parallel job per Playwright project (chromium, chromium-dark, mobile-chromium, firefox, webkit, mobile-webkit), each installing only the engine it needs
• Use 4 workers in CI (GitHub's public-repo runners have 4 vCPUs); local runs stay at 2
• Keep a small aggregate job named browser so the existing required status check on main still works; it passes only when every shard passes

Coverage is unchanged: every test still runs on every browser project.

🤖 Generated with Claude Code

https://claude.ai/code/session_014hemhbrLw8SD8kkR1hdxMK