[6/7] React migration: lazy UserProfile route

[devanshi-gpta]

Summary

Ports src/app/user/ to components/user/UserProfile.tsx and adds the lazy /user/:id route, mirroring the Angular UserModule lazy load. Fetch/abort and error handling follow the same pattern as Feed.

Worth knowing before reviewing the runtime behavior: the upstream endpoint this view depends on, https://node-hnapi.herokuapp.com/user/:id, no longer exists and responds 404 with an HTML body. That is equally true of the Angular version — the URL is unchanged — so the view renders its error state rather than a profile until the app moves to a different API. The API module rejects on !response.ok, so the failure surfaces as the component's error message instead of a SyntaxError from parsing HTML as JSON.

Proposed fixes

• Add UserProfile.tsx / UserProfile.scss.
• Add the lazy /user/:id route to App.

Link to Devin session: https://app.devin.ai/sessions/a3a0ff57bab64e25860349a003a282ef
Open in Devin Desktop: https://app.devin.ai/desktop/session/a3a0ff57bab64e25860349a003a282ef?variant=devin
Requested by: @devanshi-gpta

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