Cleanup

• Consolidated duplicated types, constants, and patterns into shared modules
• Broke apart the two largest files into smaller, focused modules
• Split the monolithic step execution switch into 10 individual handler files
• Extracted reusable UI component for CategoryBrowser
• Extracted canvas pan/zoom logic out of FlowsPage into its own hook
• Fixed a bug where inserting, moving, or cloning steps mid-flow didn’t properly wire up connections (steps got orphaned)

Play with the updates here:
https://proto-workflow-builder-mk1-git-bs-cleanup-260303-voxie.vercel.app/flows