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/src/core/config/index.ts

@@ -4,7 +4,9 @@ export function getBackendBaseURL() {
   if (env.NEXT_PUBLIC_BACKEND_BASE_URL) {
     return env.NEXT_PUBLIC_BACKEND_BASE_URL;
   } else {
-    return "http://localhost:8000";
+    // Use empty string for same-origin requests through nginx
+    // Paths like /api/models will be handled by nginx proxy
+    return "";
   }
 }
 
@@ -12,6 +14,11 @@ export function getLangGraphBaseURL() {
   if (env.NEXT_PUBLIC_LANGGRAPH_BASE_URL) {
     return env.NEXT_PUBLIC_LANGGRAPH_BASE_URL;
   } else {
-    return getBackendBaseURL();
+    // LangGraph SDK requires a full URL, construct it from current origin
+    if (typeof window !== "undefined") {
+      return `${window.location.origin}/api/langgraph`;
+    }
+    // Fallback for SSR
+    return "http://localhost:2026/api/langgraph";
   }
 }