Add ChartJS library support to AI assistant

- Detect chart library (Google Charts, ChartJS, DataTable) in Sidebar
- Pass library type through AJAX to AI module
- Hide AI assistant for DataTable library (only supports tables)
- Create ChartJS-specific configuration examples with nested plugins structure
- Update AI system prompts to generate correct JSON format per library
- Google Charts uses flat structure: {"legend": {"position": "bottom"}}
- ChartJS uses nested structure: {"plugins": {"legend": {"position": "bottom"}}}
- Add detailed ChartJS options for pie, line, bar, column, and area charts
- Update all AI provider methods (OpenAI, Gemini, Claude) to support library parameter

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: vytisbulkevicius