Implement syntax highlighting for code blocks in responses

[anshul23102]

Fixes #1140

Summary

Implements syntax highlighting for fenced code blocks in responses using highlight.js and marked.js. Code snippets now display with proper syntax coloring for improved readability, with automatic language detection and theme-aware styling.

Changes

• Library Integration: Added marked.js for markdown parsing and highlight.js for syntax highlighting (already partially present)
• Syntax Highlighting: Implemented renderMarkdownWithSyntaxHighlight() function that:
  • Parses markdown with language-aware code blocks
  • Auto-detects language from fence tags (e.g., ```python)
  • Falls back to plaintext for unrecognized languages
  • Applies highlight.js styles automatically
• Theme Support: Code blocks respect light/dark theme switching via data-theme attribute
• CSS Enhancements:
  • Extended styling to all <pre><code> blocks (not just specific containers)
  • Maintains consistent padding, font-family, and overflow handling
  • Proper background colors for both light and dark themes
• Backward Compatibility: Preserves existing .issue-snippet and .suggest-example styling

Implementation Details

• Language packs are bundled with highlight.js CDN for automatic language support
• Renderer configured to generate <pre><code class="language-xxx hljs"> structure
• window.highlightAllCode() can be called after dynamic content rendering
• window.renderMarkdownWithSyntaxHighlight() exposed globally for markdown rendering

Testing

• Verify code blocks render with syntax highlighting
• Test language detection (Python, JavaScript, Java, etc.)
• Confirm theme switching (light/dark) updates code block colors
• Check fallback behavior for unknown languages

[anshul23102]

Thank you for reviewing this PR!

This implementation adds comprehensive syntax highlighting support for code blocks in responses using highlight.js and marked.js.

When you have a moment, please review the changes and consider adding the following labels:

• gssoc-approved - to mark this as a GSSoC contribution
• frontend - for frontend changes
• enhancement - as this adds new functionality
• priority: high - if this should be prioritized

Thank you!