Add CONTRIBUTING.md for contributor guidelines

[Elshayib]

Documentation Improvement Request

Issue

The project "flatrun/ui" is a Vue.js + TypeScript dashboard for managing deployments, containers, and system resources with a comprehensive README covering features, installation, configuration, troubleshooting, and UI screenshots. However, it lacks a CONTRIBUTING.md file.

Suggested Action

Add a CONTRIBUTING.md with:

• How to report bugs (with deployment errors, container management issues, API connection problems)
• How to suggest features (new resource monitors, deployment workflows, UI improvements)
• Code style guidelines (Vue 3 Composition API, TypeScript strict, Tailwind CSS, Pinia, ESLint/Prettier)
• Pull request process
• Development setup (Node.js, pnpm/yarn, Vue DevTools, backend agent connection)
• Testing requirements (component tests, E2E with Playwright/Cypress, API integration)
• Documentation update expectations (README, component docs, API client docs)
• Security reporting process (JWT auth, API keys, container access)

This is a container management dashboard - contributions likely involve Vue components, TypeScript API clients, deployment workflows, and real-time resource monitoring.