UI Fixes: Visualization and Tool History Improvements (#323)

* fix: UI improvements for visualizations and tool history - Remove mock Mermaid visualization and use only Neo4j database - Fix close button functionality in tool history modal - Improve tool history result display with proper newline formatting - Update visualization error handling to require Neo4j connection

* fix: Tool history statistics display - Fixed field name mismatches between API and frontend - Success rate now calculated correctly from successful/total invocations - Average duration uses correct field name (average_duration_ms) - Most used tool handles empty tools list properly

* fix: Modal close button functionality in tool history - Change CSS class from 'modal-close' to 'close' to match base template expectations - Update CSS styling to use correct class name - Close button now properly triggers hideModal function from base template

Author: plusplusoneplusplus