Generate baseline screenshots (macOS/Chromium) for the elevation-chart
visual regression suite. The CI update-visual-snapshots workflow will
overwrite these with Linux variants on the first run.
Also fix the test:visual:update script: --update-snapshots is not a
valid Vitest 4 flag; the correct short form is -u.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>