chore: initial Next.js template scaffold

Next.js 16 + React 19 + TypeScript 5 + Tailwind CSS 4 + shadcn/ui.
Includes layout components, pages (home, 404, error, loading),
hooks, types, and documentation.

CONTRIBUTING.md

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+# Contributing
+
+Thank you for considering contributing to this template.
+
+## How to Contribute
+
+### Reporting Issues
+
+If you find a problem with the default configuration or have a suggestion for improvement, please open an issue.
+
+### Submitting Changes
+
+1. Fork the repository
+2. Create your feature branch: `git checkout -b feat/your-feature`
+3. Commit your changes: `git commit -m "feat: add your feature"`
+4. Push to the branch: `git push origin feat/your-feature`
+5. Open a Pull Request
+
+### Commit Message Convention
+
+Please follow the [Conventional Commits](https://www.conventionalcommits.org/) format:
+
+- `feat:` — new feature
+- `fix:` — bug fix
+- `docs:` — documentation changes
+- `chore:` — build or tooling changes
+- `refactor:` — code refactoring
+
+## Template Maintenance Principles
+
+- Keep dependencies up to date with the latest stable versions.
+- Default configuration should be minimal — only include what most projects need.
+- Avoid adding business-specific code or dependencies.
+- Components follow the shadcn/ui convention: copy source, customise locally.