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>

frontend/src/App.tsx

@@ -1,10 +1,7 @@
 import { Routes, Route, Navigate } from 'react-router-dom';
 import { useAuthStore } from './store/auth';
-
-// Placeholder components - will be replaced
-const LoginPage = () => <div>Login Page</div>;
-const DashboardPage = () => <div>Dashboard</div>;
-const MainLayout = ({ children }: { children: React.ReactNode }) => <div>{children}</div>;
+import Login from './pages/Login';
+import MainLayout from './layouts/MainLayout';
 
 function PrivateRoute({ children }: { children: React.ReactNode }) {
   const isAuthenticated = useAuthStore((state) => state.isAuthenticated);
@@ -14,17 +11,12 @@ function PrivateRoute({ children }: { children: React.ReactNode }) {
 export default function App() {
   return (
     <Routes>
-      <Route path="/login" element={<LoginPage />} />
+      <Route path="/login" element={<Login />} />
       <Route
         path="/*"
         element={
           <PrivateRoute>
-            <MainLayout>
-              <Routes>
-                <Route path="/" element={<Navigate to="/dashboard" replace />} />
-                <Route path="/dashboard" element={<DashboardPage />} />
-              </Routes>
-            </MainLayout>
+            <MainLayout />
           </PrivateRoute>
         }
       />