Implement simple blog with Next.js

[jazferlex]

Summary

• Implemented a small blog app inside the fullstack directory using Next.js (App Router + TypeScript).
• Includes pages, API routes, sample data, and basic styling.

What I added

• fullstack/data/posts.ts — sample posts array
• fullstack/app/page.tsx — home page (list of posts)
• fullstack/app/posts/[id]/page.tsx — dynamic post page (awaits params)
• fullstack/app/api/posts/route.ts — GET all posts
• fullstack/app/api/posts/[id]/route.ts — GET post by id
• fullstack/components/ThemeToggle.tsx — client theme toggle component
• fullstack/app/globals.css — basic styles
• fullstack/layout.tsx — app layout wiring

Testing notes

• API endpoints return JSON for all posts and by id.
• Dynamic route bug (params is a Promise) was fixed by awaiting params in app/posts/[id]/page.tsx.