Improve Markdown rendering

Enhance the Markdown rendering engine in 120 AI Chat to fully support advanced markdown elements like tables, multi-line code blocks, nested lists, and block-level formatting.

• Full table support with proper column alignment, borders, and scrollable overflow handling on small screens

• Syntax-highlighted code blocks with language detection and optional copy button

• Improved list rendering (support for nested bullet points and checklists with correct indentation)

• Better blockquote styling with visual distinction and nesting support