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/LoginPage.tsx

@@ -0,0 +1,86 @@
+import { useEffect } from 'react';
+import { useNavigate } from 'react-router-dom';
+import { useAuth } from '../contexts/AuthContext';
+import './LoginPage.css';
+
+const API_BASE_URL = import.meta.env.VITE_API_URL || "http://localhost:8080/api";
+
+function LoginPage() {
+  const { user, loading } = useAuth();
+  const navigate = useNavigate();
+
+  useEffect(() => {
+    if (!loading && user) {
+      navigate('/');
+    }
+  }, [user, loading, navigate]);
+
+  const handleGoogleLogin = () => {
+    window.location.href = `${API_BASE_URL}/auth/google`;
+  };
+
+  const handleGitHubLogin = () => {
+    window.location.href = `${API_BASE_URL}/auth/github`;
+  };
+
+  if (loading) {
+    return (
+      <div className="login-page">
+        <div className="login-container">
+          <p>Loading...</p>
+        </div>
+      </div>
+    );
+  }
+
+  return (
+    <div className="login-page">
+      <div className="login-container">
+        <h1 className="login-title">Realtime Collab</h1>
+        <p className="login-subtitle">Collaborate in real-time with your team</p>
+
+        <div className="login-buttons">
+          <button
+            className="login-button google-button"
+            onClick={handleGoogleLogin}
+          >
+            <svg className="button-icon" viewBox="0 0 24 24">
+              <path
+                fill="currentColor"
+                d="M22.56 12.25c0-.78-.07-1.53-.2-2.25H12v4.26h5.92c-.26 1.37-1.04 2.53-2.21 3.31v2.77h3.57c2.08-1.92 3.28-4.74 3.28-8.09z"
+              />
+              <path
+                fill="currentColor"
+                d="M12 23c2.97 0 5.46-.98 7.28-2.66l-3.57-2.77c-.98.66-2.23 1.06-3.71 1.06-2.86 0-5.29-1.93-6.16-4.53H2.18v2.84C3.99 20.53 7.7 23 12 23z"
+              />
+              <path
+                fill="currentColor"
+                d="M5.84 14.09c-.22-.66-.35-1.36-.35-2.09s.13-1.43.35-2.09V7.07H2.18C1.43 8.55 1 10.22 1 12s.43 3.45 1.18 4.93l2.85-2.22.81-.62z"
+              />
+              <path
+                fill="currentColor"
+                d="M12 5.38c1.62 0 3.06.56 4.21 1.64l3.15-3.15C17.45 2.09 14.97 1 12 1 7.7 1 3.99 3.47 2.18 7.07l3.66 2.84c.87-2.6 3.3-4.53 6.16-4.53z"
+              />
+            </svg>
+            Sign in with Google
+          </button>
+
+          <button
+            className="login-button github-button"
+            onClick={handleGitHubLogin}
+          >
+            <svg className="button-icon" viewBox="0 0 24 24">
+              <path
+                fill="currentColor"
+                d="M12 2A10 10 0 0 0 2 12c0 4.42 2.87 8.17 6.84 9.5.5.08.66-.23.66-.5v-1.69c-2.77.6-3.36-1.34-3.36-1.34-.46-1.16-1.11-1.47-1.11-1.47-.91-.62.07-.6.07-.6 1 .07 1.53 1.03 1.53 1.03.87 1.52 2.34 1.07 2.91.83.09-.65.35-1.09.63-1.34-2.22-.25-4.55-1.11-4.55-4.92 0-1.11.38-2 1.03-2.71-.1-.25-.45-1.29.1-2.64 0 0 .84-.27 2.75 1.02.79-.22 1.65-.33 2.5-.33.85 0 1.71.11 2.5.33 1.91-1.29 2.75-1.02 2.75-1.02.55 1.35.2 2.39.1 2.64.65.71 1.03 1.6 1.03 2.71 0 3.82-2.34 4.66-4.57 4.91.36.31.69.92.69 1.85V21c0 .27.16.59.67.5C19.14 20.16 22 16.42 22 12A10 10 0 0 0 12 2z"
+              />
+            </svg>
+            Sign in with GitHub
+          </button>
+        </div>
+      </div>
+    </div>
+  );
+}
+
+export default LoginPage;