Set Up Tailwind Infrastructure

[yeevon]

Summary:
Add Tailwind CSS to the Next.js project to enable utility-first styling for all pages.

Acceptance Criteria:

1. tailwindcss, postcss, and autoprefixer installed and configured.
2. tailwind.config.(js|ts) exists with content paths covering app//*, components//, and pages/**/ (if any).
3. postcss.config.(js|cjs) present and wired.
4. Global CSS includes Tailwind base, components, utilities (e.g., globals.css with @tailwind base; @tailwind components; @tailwind utilities;).
5. Project builds and runs; Tailwind classes render in a sample element on the homepage.