[UI] GTFS Diff Display POC

[cka-y]

Describe the problem

There is no visual interface to explore the output of the gtfs_diff engine. Before wiring up a real backend, we can validate the schema with our first consumer, which is our own UI.

Proposed solution

Build a React POC that renders a hardcoded changelog following the schema defined in MobilityData/gtfs_diff#10. The goal is to validate the schema and data presentation before any real backend integration.

Alternatives considered

• Build the UI after backend integration: delays schema validation and layout decisions, risks rework if the data shape does not translate well visually.