feature: PR 2/8 — models, Promise-based HN API and comment util

[devin-ai-integration]

Summary

PR 2 of 8 (stacked on #698). Framework-agnostic foundations under src/react/; nothing under src/app/ is touched.

• src/react/models/* — the six Angular models ported verbatim (hence strictPropertyInitialization: false in the React tsconfig, since the originals are classes with uninitialized fields).
• src/react/utils/comments.ts — formatComments(count) replaces CommentPipe, same output (discuss / 1 comment / N comments).
• src/react/api/hackernews.ts — replaces HackerNewsAPIService: RxJS Observable + unfetch become platform fetch + Promises.

Two deliberate deviations from the Angular service:

1. HTTP errors now reject. The old lazyFetch resolved on any response, so a 500 surfaced as a JSON parse failure or an empty feed; the shared getJson helper throws on !res.ok so Feed/ItemDetails can render ErrorMessage.
2. fetchItemContent awaits the poll fetches instead of returning early:

if (story.type === 'poll') {
    story.poll_votes_count = 0;
    const results = await Promise.all(story.poll.map((_, i) => fetchPollContent(story.id + i + 1)));
    results.forEach((r, i) => { story.poll[i] = r; story.poll_votes_count += r.points; });
}
return story; // poll + poll_votes_count fully populated

The Angular version returned the story while subscribe callbacks mutated poll afterwards — fine with Angular change detection, unreliable with React state.

Also note FeedType ('poll' | 'story' | 'job') is the item type, not the route name, so feed names get their own FeedName type used by fetchFeed and, later, the routes.

Verified: npm run typecheck:react, npx vite build, plus a live fetchFeed('news', 1) call returning 30 stories.

Devin-Org: engineering

Link to Devin session: https://app.devin.ai/sessions/e6419ebe67ff4f749fe1573a8e26cd07
Requested by: @abboudp

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Devin errored when opening this Pull Request as abboudp.
As a fallback, Devin opened this PR as itself.

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