Replace generic Vue template README with Connect 4-specific documentation

[Copilot]

The frontend README contained boilerplate Vue/Vite template content. Updated to document the actual Connect 4 game application.

Changes

• Project identity: Changed title and description to reflect Connect 4 Agent frontend
• Features section: Documents interactive game board, visual feedback (winning tiles, last move indicator, potential moves), game state management, and backend integration
• Prerequisites: Added Node.js, npm, and backend server requirements
• Code quality commands: Added npm run format and npm run type-check to development workflow
• Project structure: Maps key files (ConnectFour.vue, App.vue) to their purposes
• Removed: Verbose TypeScript/Volar "Take Over Mode" instructions irrelevant to end users

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💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.