fix: bridge Dify design tokens for streamdown table fullscreen

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Summary

• Streamdown's table fullscreen uses createPortal to document.body, with shadcn/ui Tailwind classes (bg-background, bg-muted, border-border, etc.) that don't resolve in Dify's design system — resulting in missing background, borders, and text colors.
• Added CSS overrides for [data-streamdown="table-fullscreen"] in markdown.css, following the same pattern already used for [data-streamdown="table-wrapper"].
  fixes View fullscreen error #34182

Screenshots

Before	After

Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran make lint and make type-check (backend) and cd web && npx lint-staged (frontend) to appease the lint gods