Add proxy server configuration for Chromium

[ColumbusLabs]

Summary

• add a proxy_server browser option that forwards Chromium's --proxy-server flag
• use CHOREO_PROXY_SERVER when the explicit option is omitted or empty, including for Plotly and Kaleido callers that construct Choreographer internally
• add focused regression coverage for explicit, environment, precedence, empty, and unset configurations

Closes #301

Testing

• pytest -v tests/test_chromium.py (6 passed)
• ruff check --ignore CPY001 src/choreographer/browsers/chromium.py tests/test_chromium.py
• ruff format --check src/choreographer/browsers/chromium.py tests/test_chromium.py
• pyright
• mypy src/choreographer/browsers/chromium.py tests/test_chromium.py
• git diff --check

The full local poe test browser suite cannot complete on this macOS 26 machine because the repository-pinned Chrome 142 build exits during startup with chrome_crashpad_handler: --database is required. The same test_watchdog[headless] failure reproduces from an untouched origin/main worktree; the focused proxy tests and all non-browser checks above pass.

[camdecoster]

Thanks for the PR! I'll review and follow up.

[camdecoster]

Thanks for the update! Could you please make the requested changes and tag me for another review when you're done?

[ColumbusLabs]

Addressed the three requested changes in f2432d0: the explicit option now falls back to CHOREO_PROXY_SERVER when falsy, proxy values are no longer redacted in logs, and the README section has been removed. I also updated the focused tests for None and empty-string fallback behavior. @camdecoster, this is ready for another review.