Add standalone TypeScript interfaces for React migration (Phase 1)

Summary

Converts Angular class-based models from src/app/shared/models/ into standalone TypeScript interfaces under a new src/types/ directory for use by the React app. This is Phase 1 of a multi-phase migration — only type definitions, no behavioral changes.

Types added: Story, Comment, User, PollResult, FeedType, Settings

All types are re-exported from src/types/index.ts as a barrel file.

No existing files were modified.

Review & Testing Checklist for Human

• time_ago type inconsistency: Story.time_ago is number but Comment.time_ago is string. This matches the original Angular models, but verify against the actual HN API to confirm this is intentional and not a latent bug being carried forward.
• User.crated_time typo: The field name crated_time is carried over from the original Angular model. Decide whether to fix it to created_time now (breaking change vs API shape) or leave it for a later PR.
• No optional fields: All interface properties are required. Verify whether the HN API always returns every field (e.g., url, poll, domain may be absent on certain item types). Consider adding ? where appropriate before the React app consumes these types.

Notes

• This PR is additive only (7 new files, 0 modified) — no risk to existing Angular app behavior.
• Recommended test plan: run npx tsc --noEmit on the src/types/ files to confirm they compile cleanly.

Requested by: @eashansinha
Link to Devin run