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>

app/(dashboard)/layout.tsx

@@ -3,6 +3,7 @@
 import { Header } from '@/components/layout/Header';
 import { Sidebar } from '@/components/layout/Sidebar';
 import { useUIStore } from '@/stores/ui-store';
+import { AuthGuard } from '@/components/providers/AuthGuard';
 
 export default function DashboardLayout({
   children,
@@ -12,18 +13,20 @@ export default function DashboardLayout({
   const sidebarOpen = useUIStore((state) => state.sidebarOpen);
 
   return (
-    <div className="min-h-screen">
-      <Header />
-      <div className="flex">
-        <Sidebar />
-        <main
-          className={`flex-1 transition-all duration-200 ${
-            sidebarOpen ? 'ml-64' : 'ml-0'
-          }`}
-        >
-          <div className="p-6">{children}</div>
-        </main>
+    <AuthGuard>
+      <div className="min-h-screen">
+        <Header />
+        <div className="flex">
+          <Sidebar />
+          <main
+            className={`flex-1 transition-all duration-200 ${
+              sidebarOpen ? 'ml-64' : 'ml-0'
+            }`}
+          >
+            <div className="p-6">{children}</div>
+          </main>
+        </div>
       </div>
-    </div>
+    </AuthGuard>
   );
 }