fix: Markdown code block contrast in dark mode (fixes #1253)

[spider-yamet]

Summary

Fixes unreadable code blocks in dark mode (dark text on dark background) and ensures code blocks use theme-aware foreground color everywhere.

Fixes #1253

Changes

Dark mode code block contrast

• src/style/markdown-styles.css: In [data-theme='dark'], set explicit text color for code:
  • .markdown-body pre and .markdown-body pre code: color: #e6edf3
  • .markdown-body code (inline): color: #e6edf3
• Fixes the case where app theme is dark but OS preference is light: github-markdown-css only uses prefers-color-scheme, so --fgColor-default stayed dark and code was unreadable on dark backgrounds.

Theme token for code foreground

• src/style/token.css: In [data-theme='dark'], set --code-foreground: #e6edf3 so code text follows app theme and stays readable on --code-surface.

WorkFlow MarkDown

• src/components/WorkFlow/MarkDown.tsx: Applied text-code-foreground to pre and code (block and inline) so workflow markdown code blocks use the semantic token and stay readable in dark mode.

Testing

• Dark mode: Code blocks in chat/README and workflow markdown show light text (#e6edf3) on dark background.
• Light mode: Unchanged; code remains dark on light background.

[bytecii]

Can you also add a snapshot?

[Wendong-Fan]

thanks @spider-yamet for the contribution! Could you add a short video or snapshot for your PR like how #1258 did? Thanks!