CRITICAL FIX: Resolved 'Failed to fetch' errors in overlay templates

API CONNECTIVITY ENHANCEMENTS:
- Modified fixtures template JavaScript to use configurable webServerBaseUrl instead of relative URLs
- Enhanced Qt player WebChannel system to pass web server base URL (http://127.0.0.1:5001) to all overlay templates
- Updated PlayerWindow._get_web_server_base_url() method to provide consistent API endpoint configuration
- Enhanced QtVideoPlayer._get_web_server_base_url() method for cross-component consistency
- Modified all overlay data updates (play_video, update_overlay_periodically, default overlay loading) to include webServerBaseUrl

TEMPLATE SYSTEM IMPROVEMENTS:
- Fixed API request URLs in fixtures template from relative (/api/endpoint) to absolute (http://127.0.0.1:5001/api/endpoint)
- Enhanced template initialization to receive web server configuration via WebChannel updateOverlayData()
- Improved fallback mechanism with proper API endpoint resolution

ROOT CAUSE RESOLUTION:
- Templates loaded via local schemes (file://, qrc://) cannot resolve relative API URLs
- WebChannel system now provides absolute web server URLs to enable proper API communication
- All overlay templates now have access to webServerBaseUrl for API requests

This fix ensures Qt player overlay templates can successfully communicate with the Flask web dashboard API endpoints, eliminating 'TypeError: Failed to fetch' errors and enabling proper data display.