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/pages/AuthCallback.tsx

@@ -0,0 +1,68 @@
+import { useEffect, useState } from 'react';
+import { useNavigate, useSearchParams } from 'react-router-dom';
+import { useAuth } from '../contexts/AuthContext';
+
+function AuthCallback() {
+  const [searchParams] = useSearchParams();
+  const navigate = useNavigate();
+  const { login } = useAuth();
+  const [error, setError] = useState<string | null>(null);
+
+  useEffect(() => {
+    const handleCallback = async () => {
+      const token = searchParams.get('token');
+      const redirect = searchParams.get('redirect') || '/';
+
+      if (!token) {
+        setError('No authentication token received');
+        setTimeout(() => navigate('/login'), 2000);
+        return;
+      }
+
+      try {
+        await login(token);
+        navigate(redirect);
+      } catch (err) {
+        console.error('Login error:', err);
+        setError('Authentication failed. Please try again.');
+        setTimeout(() => navigate('/login'), 2000);
+      }
+    };
+
+    handleCallback();
+  }, [searchParams, login, navigate]);
+
+  return (
+    <div style={{
+      display: 'flex',
+      alignItems: 'center',
+      justifyContent: 'center',
+      minHeight: '100vh',
+      background: 'linear-gradient(135deg, #667eea 0%, #764ba2 100%)',
+    }}>
+      <div style={{
+        background: 'white',
+        borderRadius: '16px',
+        padding: '48px',
+        textAlign: 'center',
+      }}>
+        {error ? (
+          <>
+            <h2 style={{ color: '#e53e3e', marginBottom: '8px' }}>Error</h2>
+            <p style={{ color: '#718096' }}>{error}</p>
+            <p style={{ color: '#718096', fontSize: '14px', marginTop: '16px' }}>
+              Redirecting to login...
+            </p>
+          </>
+        ) : (
+          <>
+            <h2 style={{ color: '#1a202c', marginBottom: '8px' }}>Logging you in...</h2>
+            <p style={{ color: '#718096' }}>Please wait while we complete the authentication.</p>
+          </>
+        )}
+      </div>
+    </div>
+  );
+}
+
+export default AuthCallback;