feat: photo-search, system settings, ROADMAP

- PhotoSearchModal: Yandex photo-search с профилями доменов
- SystemSettings: управление app_settings (admin-only, /system)
- ROADMAP.md: актуальный план фич P1-P10
- Header, ChannelView, session: поддержка is_admin

components/Header.js

@@ -1,7 +1,7 @@
 'use client';
 import Link from 'next/link';
 import { useRouter } from 'next/navigation';
-import { Sparkles, LogOut } from 'lucide-react';
+import { Sparkles, LogOut, Settings2 } from 'lucide-react';
 import ThemeToggle from './ThemeToggle';
 
 export default function Header({ user }) {
@@ -18,6 +18,16 @@ export default function Header({ user }) {
           <span className="font-bold">ZeroPost</span>
         </Link>
         <div className="flex items-center gap-2">
+          {user?.isAdmin && (
+            <Link
+              href="/system"
+              className="btn-ghost p-2 text-sm hidden sm:inline-flex"
+              title="Системные настройки"
+            >
+              <Settings2 className="w-4 h-4" />
+              <span className="hidden md:inline">Система</span>
+            </Link>
+          )}
           <span className="text-sm text-gray-500 hidden sm:inline mr-2">{user?.email}</span>
           <ThemeToggle />
           <button onClick={logout} className="btn-ghost p-2" title="Выйти">