feat: add unified development environment with nginx proxy

Add a root-level Makefile to manage frontend, backend, and nginx services:
- `make check` validates required dependencies (Node.js 22+, pnpm, uv, nginx)
- `make install` installs all project dependencies
- `make dev` starts all services with unified port 2026
- `make stop` and `make clean` for cleanup

Update nginx configuration:
- Change port from 8000 to 2026
- Add frontend upstream and routing (port 3000)
- Add /api/langgraph/* routing with path rewriting to LangGraph server
- Keep other /api/* routes to Gateway API
- Route non-API requests to frontend

Update frontend configuration:
- Use relative URLs through nginx proxy by default
- Support environment variables for direct backend access
- Construct full URL for LangGraph SDK compatibility

Clean up backend Makefile by removing nginx and serve targets.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

frontend/.env.example

@@ -9,8 +9,11 @@
 # When adding additional environment variables, the schema in "/src/env.js"
 # should be updated accordingly.
 
-# The base URL of DeerFlow's backend service
-NEXT_PUBLIC_BACKEND_BASE_URL="http://localhost:8000"
+# Backend API URLs (optional)
+# Leave these commented out to use the default nginx proxy (recommended for `make dev`)
+# Only set these if you need to connect to backend services directly
+# NEXT_PUBLIC_BACKEND_BASE_URL="http://localhost:8001"
+# NEXT_PUBLIC_LANGGRAPH_BASE_URL="http://localhost:2024"
 
 # Better Auth
 ## Better Auth - Google OAuth