feat: admin back-office system + AI assistant with action commands

- Admin analytics (recharts charts, overview stats, trend analysis, time range selector)
- Admin permissions (AdminRole/AdminUser models, role CRUD, permission catalog)
- Admin system config (site/AI/member category tabs, per-key save)
- Admin operations (Banner CRUD, push notification send/delete)
- Admin user management (table, search, create/edit, ban/delete)
- Admin layout (custom top bar with branding + sidebar, admin AI assistant)
- Admin login (adminToken localStorage, adminInfo display)
- Admin AI assistant (purple '运营助手', context-aware prompts, action commands)
- Public AI assistant Phase B (action commands: navigate, setModel, startChat, openSkill, setParameter)
- Assistant context mapping + action executor
- Skills API fix: tags parsing fallback (JSON.parse → split)
- Sandbox crash fix: curScene fallback system prompt
- JWT token expiration: access 2h→7d, refresh 7d→30d, admin 8h→7d
- i18n: assistant-related translations (zh/en)
- PM2 ecosystem config for process management
- AI assistant login prompt fix: admin uses getAdminToken(), public uses apiFetch with token refresh

frontend/src/app/sandbox/page.tsx

@@ -147,8 +147,9 @@ function SandboxPage() {
       let reply = '';
       if (tk) {
         const curScene = SCENES.find(s => s.id === scene) || SCENES[0];
+        const systemPrompt = curScene?.systemPrompt || '你是一个智能 AI 助手';
         const apiMessages = [
-          { role: 'system', content: curScene.systemPrompt },
+          { role: 'system', content: systemPrompt },
           ...messages,
           userMsg,
         ].map(m => ({ role: m.role, content: m.content }));
@@ -433,7 +434,7 @@ function SandboxPage() {
             <div ref={messagesContainerRef} className="flex-1 overflow-y-auto p-4 space-y-4">
               {isNewChat && (
                 <div className="flex flex-wrap gap-2 mb-4">
-                  {currentScene.starters.map((q, i) => (
+                  {currentScene?.starters?.map((q, i) => (
                     <button key={i} onClick={() => setInput(q)}
                       className="px-3 py-1.5 text-xs bg-muted text-muted-foreground rounded-full border border-border hover:bg-accent hover:text-foreground transition-colors">
                       {q}