Frontend: Complete Next.js implementation

Core Features:
- ✅ Authentication (login/register pages)
- ✅ Main layout with sidebar
- ✅ Channel list and navigation
- ✅ Channel page with real-time messaging
- ✅ Direct messages (DMs) page
- ✅ Message components (list, input)
- ✅ WebSocket integration for real-time updates
- ✅ Typing indicators
- ✅ Online status indicators
- ✅ Stores (auth, channels, messages)
- ✅ API client with JWT auth
- ✅ Socket.IO client wrapper
- ✅ Responsive UI with Tailwind

Tech Stack:
- Next.js 14 (App Router)
- TypeScript
- TailwindCSS
- Socket.IO client
- Zustand (state management)
- Axios (HTTP client)
- date-fns (date formatting)
- lucide-react (icons)

Build Status: ✅ Successfully compiles
Production Ready: ✅ Optimized build generated

frontend/next-env.d.ts

@@ -0,0 +1,5 @@
+/// <reference types="next" />
+/// <reference types="next/image-types/global" />
+
+// NOTE: This file should not be edited
+// see https://nextjs.org/docs/app/building-your-application/configuring/typescript for more information.