feat(frontend): Implement complete authentication system

Implemented comprehensive JWT-based authentication with token refresh mechanism, user state management, and protected routes.

Changes:
- Upgraded API client from fetch to Axios with automatic token refresh interceptors
- Created API configuration with centralized endpoint definitions
- Implemented Zustand auth store for user state management with persistence
- Created React Query hooks for login, register, logout, and current user
- Built login and registration pages with form validation (Zod + React Hook Form)
- Implemented AuthGuard component for route protection
- Enhanced Header with user dropdown menu and logout functionality
- Updated Sidebar with user information display at bottom
- Added Team navigation item to sidebar
- Configured environment variables for API base URL

Technical Details:
- JWT token storage in localStorage with secure key names
- Automatic token refresh on 401 responses
- Request queueing during token refresh to prevent race conditions
- TypeScript strict typing throughout
- ESLint compliant code (fixed type safety issues)
- Proper error handling with user-friendly messages

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

lib/api/config.ts

@@ -0,0 +1,23 @@
+export const API_BASE_URL = process.env.NEXT_PUBLIC_API_URL || 'http://localhost:5000';
+
+export const API_ENDPOINTS = {
+  // Auth
+  LOGIN: '/api/auth/login',
+  REGISTER_TENANT: '/api/auth/register-tenant',
+  REFRESH_TOKEN: '/api/auth/refresh',
+  LOGOUT: '/api/auth/logout',
+  ME: '/api/auth/me',
+
+  // Users
+  USERS: '/api/users',
+  USER_PROFILE: (userId: string) => `/api/users/${userId}`,
+
+  // Tenants
+  TENANT_USERS: (tenantId: string) => `/api/tenants/${tenantId}/users`,
+  ASSIGN_ROLE: (tenantId: string, userId: string) =>
+    `/api/tenants/${tenantId}/users/${userId}/role`,
+
+  // Projects (to be implemented)
+  PROJECTS: '/api/projects',
+  PROJECT: (id: string) => `/api/projects/${id}`,
+};