URL Source: response caching & offline mode

- Persist the last successful response to disk per saved request
- View cached responses when offline or the endpoint is unavailable
- Show a "cached" indicator with the last-fetch timestamp

Part of #68.

No files or tests are named in the issue. Start by locating the URL Source and saved-request entry points in the Rust desktop application, then trace response persistence and fetch-status handling. Done means successful responses are cached per saved request, available offline or after endpoint failure, and display a cached indicator with the last-fetch timestamp.

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