Add proxy server configuration for Chromium

[ColumbusLabs]

Summary

• add a proxy_server browser option that forwards Chromium’s --proxy-server flag
• add a CHOREO_PROXY_SERVER environment fallback for Plotly and Kaleido callers that construct Choreographer internally
• redact authenticated proxy URLs from constructor and CLI logs
• document both configuration paths and add focused regression coverage

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
• uv build

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.