feat: Implement Share Modal for document sharing functionality

- Added ShareModal component to manage user and link sharing for documents.
- Created AuthContext to handle user authentication state and token management.
- Updated useYjsDocument hook to support sharing via tokens.
- Enhanced Yjs document creation to include user information and authentication tokens.
- Introduced AuthCallback page to handle authentication redirects and token processing.
- Modified EditorPage and KanbanPage to include share functionality.
- Created LoginPage with Google and GitHub authentication options.
- Added styles for LoginPage.
- Defined types for authentication and sharing in respective TypeScript files.

frontend/src/api/client.ts

@@ -0,0 +1,31 @@
+const API_BASE_URL = import.meta.env.VITE_API_URL || "http://localhost:8080/api";
+
+export async function authFetch(url: string, options?: RequestInit): Promise<Response> {
+  const token = localStorage.getItem('auth_token');
+
+  const headers: HeadersInit = {
+    'Content-Type': 'application/json',
+    ...options?.headers,
+  };
+
+  // Add Authorization header if token exists
+  if (token) {
+    headers['Authorization'] = `Bearer ${token}`;
+  }
+
+  const response = await fetch(url, {
+    ...options,
+    headers,
+  });
+
+  // Handle 401: Token expired or invalid
+  if (response.status === 401) {
+    localStorage.removeItem('auth_token');
+    window.location.href = '/login';
+    throw new Error('Unauthorized');
+  }
+
+  return response;
+}
+
+export { API_BASE_URL };