Add screenshots for marketing example MCP Apps

Screenshots for the following examples:
- budget-allocator-server
- cohort-heatmap-server
- customer-segmentation-server
- scenario-modeler-server
- system-monitor-server

These will be hosted via GitHub Pages and referenced in the
example READMEs, keeping binary assets out of the main branch.

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

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

Author: jonathanhefner