feat: Add authentication system and admin panel

- Add User model with authentication
- Create JWT-based auth service
- Add login page with admin/admin default credentials
- Create admin panel with user management, settings, and site configuration
- Move settings to admin section (admin only)
- Change search page from multi-site checkboxes to single site selector
- Add logout functionality to layout
- Protect routes with authentication

frontend/src/hooks/use-auth.js

@@ -0,0 +1,101 @@
+import { createContext, useContext, useState, useEffect } from 'react';
+import axios from 'axios';
+
+const AuthContext = createContext(null);
+
+const API_URL = '/api/auth';
+
+export function AuthProvider({ children }) {
+    const [user, setUser] = useState(null);
+    const [loading, setLoading] = useState(true);
+
+    // Check for existing token on mount
+    useEffect(() => {
+        const token = localStorage.getItem('token');
+        if (token) {
+            // Verify token is still valid
+            axios.get(`${API_URL}/me`, {
+                headers: { Authorization: `Bearer ${token}` }
+            })
+            .then(response => {
+                setUser({
+                    ...response.data,
+                    token
+                });
+            })
+            .catch(() => {
+                // Token invalid, clear it
+                localStorage.removeItem('token');
+            })
+            .finally(() => {
+                setLoading(false);
+            });
+        } else {
+            setLoading(false);
+        }
+    }, []);
+
+    const login = async (username, password) => {
+        const response = await axios.post(`${API_URL}/login`, {
+            username,
+            password
+        });
+
+        const { token, username: userName, is_admin } = response.data;
+        localStorage.setItem('token', token);
+
+        setUser({
+            username: userName,
+            is_admin,
+            token
+        });
+
+        return response.data;
+    };
+
+    const logout = () => {
+        localStorage.removeItem('token');
+        setUser(null);
+    };
+
+    const changePassword = async (currentPassword, newPassword) => {
+        const token = localStorage.getItem('token');
+        await axios.post(`${API_URL}/change-password`, {
+            current_password: currentPassword,
+            new_password: newPassword
+        }, {
+            headers: { Authorization: `Bearer ${token}` }
+        });
+    };
+
+    // Helper function to get auth headers for API calls
+    const getAuthHeaders = () => {
+        const token = localStorage.getItem('token');
+        return token ? { Authorization: `Bearer ${token}` } : {};
+    };
+
+    const value = {
+        user,
+        loading,
+        login,
+        logout,
+        changePassword,
+        getAuthHeaders,
+        isAuthenticated: !!user,
+        isAdmin: user?.is_admin || false
+    };
+
+    return (
+        <AuthContext.Provider value={value}>
+            {children}
+        </AuthContext.Provider>
+    );
+}
+
+export function useAuth() {
+    const context = useContext(AuthContext);
+    if (!context) {
+        throw new Error('useAuth must be used within an AuthProvider');
+    }
+    return context;
+}