fix(ui): theme dark-mode scrollbars globally

[akira69]

Summary

Applies the dark scrollbar theme to the page scrollbar as well as nested scroll areas in dark mode.

What Changed

• Extended the global dark-mode scrollbar selectors in the color mode context to cover html and body.
• Kept light mode on native scrollbar styling by only injecting the custom rules in dark mode.

Testing Performed

• ./node_modules/.bin/eslint src/contexts/color-mode/index.tsx
• VITE_APIURL=/api/v1 npm run build
• Playwright check on http://127.0.0.1:9875: toggled dark mode on the home page and verified page-level and nested scrollbar styling is applied
• Playwright check on http://127.0.0.1:9875: toggled back to light mode and verified the custom scrollbar styling is removed

Test Checklist

• Dark mode applies themed scrollbars to page-level and nested scroll areas
• Light mode restores native scrollbar styling