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>

stores/authStore.ts

@@ -0,0 +1,45 @@
+import { create } from 'zustand';
+import { persist } from 'zustand/middleware';
+
+export interface User {
+  id: string;
+  email: string;
+  fullName: string;
+  tenantId: string;
+  tenantName: string;
+  role: 'TenantOwner' | 'TenantAdmin' | 'TenantMember' | 'TenantGuest';
+  isEmailVerified: boolean;
+}
+
+interface AuthState {
+  user: User | null;
+  isAuthenticated: boolean;
+  isLoading: boolean;
+
+  setUser: (user: User) => void;
+  clearUser: () => void;
+  setLoading: (loading: boolean) => void;
+}
+
+export const useAuthStore = create<AuthState>()(
+  persist(
+    (set) => ({
+      user: null,
+      isAuthenticated: false,
+      isLoading: true,
+
+      setUser: (user) =>
+        set({ user, isAuthenticated: true, isLoading: false }),
+      clearUser: () =>
+        set({ user: null, isAuthenticated: false, isLoading: false }),
+      setLoading: (loading) => set({ isLoading: loading }),
+    }),
+    {
+      name: 'colaflow-auth',
+      partialize: (state) => ({
+        user: state.user,
+        isAuthenticated: state.isAuthenticated,
+      }),
+    }
+  )
+);