fix(stats): match document background to page theme - #52169

Summary

• Set the document canvas background to the same white or #161616 used by the stats page, so rubber-band scrolling does not expose a white background in dark mode.
• Follow explicit dark/light preference and system dark appearance without changing scroll behavior.

Checks

• bun typecheck in packages/stats/app
• bun run --bun build in packages/stats/app
• bun run prettier --check packages/stats/app/src/routes/index.css
• Verified computed root and page background colors match for all six combinations of light/dark/system preference and light/dark OS appearance on the real data page with the new CSS rule applied. Local development server could not load the data-backed page without its production environment, so this is a CSS-level browser check, not a full local UI comparison.

Requested by: @adamdotdevin (Adam via Slack)