feat: add Layer 3 - API Gateway main app, WhatsApp routes, Frontend pages

API Gateway:
- main.py with FastAPI app, CORS, health endpoints
- WhatsApp routes: accounts CRUD, conversations, messages, internal events
- WhatsApp schemas for request/response validation

Frontend:
- Login page with register option for first admin
- MainLayout with sidebar navigation and user dropdown
- Dashboard with statistics cards (accounts, conversations)
- WhatsApp Accounts page with QR modal for connection
- Inbox page with conversation list and real-time chat

Full feature set for Fase 1 Foundation complete.

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

services/api-gateway/app/main.py

@@ -0,0 +1,40 @@
+from fastapi import FastAPI
+from fastapi.middleware.cors import CORSMiddleware
+from app.core.config import get_settings
+from app.core.database import engine, Base
+from app.routers import auth, whatsapp
+
+settings = get_settings()
+
+# Create tables
+Base.metadata.create_all(bind=engine)
+
+app = FastAPI(
+    title="WhatsApp Centralizado API",
+    description="API Gateway for WhatsApp Centralizado platform",
+    version="1.0.0",
+)
+
+# CORS
+origins = settings.CORS_ORIGINS.split(",")
+app.add_middleware(
+    CORSMiddleware,
+    allow_origins=origins,
+    allow_credentials=True,
+    allow_methods=["*"],
+    allow_headers=["*"],
+)
+
+# Routers
+app.include_router(auth.router)
+app.include_router(whatsapp.router)
+
+
+@app.get("/health")
+def health_check():
+    return {"status": "ok"}
+
+
+@app.get("/api/health")
+def api_health_check():
+    return {"status": "ok", "service": "api-gateway"}