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/contexts/AuthContext.tsx

@@ -0,0 +1,109 @@
+import { createContext, useContext, useState, useEffect, ReactNode } from 'react';
+import type { User, AuthContextType } from '../types/auth';
+import { authApi } from '../api/auth';
+
+const AuthContext = createContext<AuthContextType | undefined>(undefined);
+
+export function AuthProvider({ children }: { children: ReactNode }) {
+  const [user, setUser] = useState<User | null>(null);
+  const [token, setToken] = useState<string | null>(null);
+  const [loading, setLoading] = useState(true);
+  const [error, setError] = useState<string | null>(null);
+
+  // Initialize auth state on mount
+  useEffect(() => {
+    const initAuth = async () => {
+      const storedToken = localStorage.getItem('auth_token');
+
+      if (!storedToken) {
+        setLoading(false);
+        return;
+      }
+
+      try {
+        setToken(storedToken);
+        const currentUser = await authApi.getCurrentUser();
+        setUser(currentUser);
+        setError(null);
+      } catch (err) {
+        console.error('Failed to validate token:', err);
+        localStorage.removeItem('auth_token');
+        setToken(null);
+        setUser(null);
+        setError('Session expired');
+      } finally {
+        setLoading(false);
+      }
+    };
+
+    initAuth();
+  }, []);
+
+  const login = async (newToken: string) => {
+    try {
+      setLoading(true);
+      setError(null);
+
+      localStorage.setItem('auth_token', newToken);
+      setToken(newToken);
+
+      const currentUser = await authApi.getCurrentUser();
+      setUser(currentUser);
+    } catch (err) {
+      console.error('Login failed:', err);
+      localStorage.removeItem('auth_token');
+      setToken(null);
+      setUser(null);
+      setError('Login failed');
+      throw err;
+    } finally {
+      setLoading(false);
+    }
+  };
+
+  const logout = () => {
+    localStorage.removeItem('auth_token');
+    setUser(null);
+    setToken(null);
+    setError(null);
+
+    // Call backend logout endpoint (fire and forget)
+    authApi.logout().catch(console.error);
+
+    // Redirect to login
+    window.location.href = '/login';
+  };
+
+  const refreshUser = async () => {
+    if (!token) return;
+
+    try {
+      const currentUser = await authApi.getCurrentUser();
+      setUser(currentUser);
+      setError(null);
+    } catch (err) {
+      console.error('Failed to refresh user:', err);
+      setError('Failed to refresh user');
+    }
+  };
+
+  const value: AuthContextType = {
+    user,
+    token,
+    loading,
+    error,
+    login,
+    logout,
+    refreshUser,
+  };
+
+  return <AuthContext.Provider value={value}>{children}</AuthContext.Provider>;
+}
+
+export function useAuth(): AuthContextType {
+  const context = useContext(AuthContext);
+  if (context === undefined) {
+    throw new Error('useAuth must be used within an AuthProvider');
+  }
+  return context;
+}