feat: add Layer 2 - WhatsApp Core logic, API Gateway models/auth, Frontend core

WhatsApp Core:
- SessionManager with Baileys integration for multi-account support
- Express server with REST API and Socket.IO for real-time events
- Session lifecycle management (create, disconnect, delete)
- Message sending with support for text, image, document, audio, video

API Gateway:
- Database models: User, WhatsAppAccount, Contact, Conversation, Message
- JWT authentication with access/refresh tokens
- Auth endpoints: login, refresh, register, me
- Pydantic schemas for request/response validation

Frontend:
- React 18 app structure with routing
- Zustand auth store with persistence
- API client with automatic token handling
- Base CSS and TypeScript declarations

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

frontend/src/api/client.ts

@@ -0,0 +1,79 @@
+const API_BASE_URL = import.meta.env.VITE_API_URL || '/api';
+
+interface RequestOptions extends RequestInit {
+  params?: Record<string, string>;
+}
+
+class ApiClient {
+  private baseUrl: string;
+
+  constructor(baseUrl: string) {
+    this.baseUrl = baseUrl;
+  }
+
+  private getAuthHeaders(): Record<string, string> {
+    const token = localStorage.getItem('access_token');
+    return token ? { Authorization: `Bearer ${token}` } : {};
+  }
+
+  private async request<T>(endpoint: string, options: RequestOptions = {}): Promise<T> {
+    const { params, ...fetchOptions } = options;
+
+    let url = `${this.baseUrl}${endpoint}`;
+    if (params) {
+      const searchParams = new URLSearchParams(params);
+      url += `?${searchParams.toString()}`;
+    }
+
+    const headers: Record<string, string> = {
+      'Content-Type': 'application/json',
+      ...this.getAuthHeaders(),
+      ...(options.headers as Record<string, string>),
+    };
+
+    const response = await fetch(url, {
+      ...fetchOptions,
+      headers,
+    });
+
+    if (response.status === 401) {
+      localStorage.removeItem('access_token');
+      localStorage.removeItem('refresh_token');
+      window.location.href = '/login';
+      throw new Error('Unauthorized');
+    }
+
+    if (!response.ok) {
+      const error = await response.json().catch(() => ({ detail: 'Request failed' }));
+      throw new Error(error.detail || 'Request failed');
+    }
+
+    return response.json();
+  }
+
+  get<T>(endpoint: string, options?: RequestOptions): Promise<T> {
+    return this.request<T>(endpoint, { ...options, method: 'GET' });
+  }
+
+  post<T>(endpoint: string, data?: unknown, options?: RequestOptions): Promise<T> {
+    return this.request<T>(endpoint, {
+      ...options,
+      method: 'POST',
+      body: JSON.stringify(data),
+    });
+  }
+
+  put<T>(endpoint: string, data?: unknown, options?: RequestOptions): Promise<T> {
+    return this.request<T>(endpoint, {
+      ...options,
+      method: 'PUT',
+      body: JSON.stringify(data),
+    });
+  }
+
+  delete<T>(endpoint: string, options?: RequestOptions): Promise<T> {
+    return this.request<T>(endpoint, { ...options, method: 'DELETE' });
+  }
+}
+
+export const apiClient = new ApiClient(API_BASE_URL);