[FEATURE] Implement markdown rendering for responses

[UncleTallest]

Goal: Render Claude's markdown responses properly (code blocks, formatting, etc.).

Tasks:

• Research GTK4 markdown rendering options
• Implement basic markdown to GTK TextBuffer
• Support code blocks with syntax highlighting
• Support bold, italic, lists
• Handle long code blocks (scrolling)
• Test with various markdown formats

Success criteria:

• Code blocks render with monospace font
• Basic formatting (bold, italic, lists) works
• Readable and follows GNOME HIG

Output: src/claude_devel_client/ui/markdown_renderer.py