feat: Track and display OpenAPI spec changes over time

[tunjiadeyemi]

Problem
When a backend team updates the OpenAPI spec, frontend consumers have no easy way to know what changed. They have to manually scan the entire document on every reload.

Proposed solution:
Add an API change history feature to Swagger UI that:

• Saves a snapshot of the spec on each load/reload (browser localStorage, keyed by spec URL)
• Diffs the new spec against the previous snapshot
• Surfaces changes in a history sidebar (accessible from a top-bar icon)

What gets detected:

• Endpoints added, removed, or modified
• Parameter, request body, and response changes

Notes:

• Client-side only, can be improved later through a backend architecture
• Unseen changes indicated by a badge on the history icon