feat(admin): add LLM call logs, system info, plugin list pages

- ai-core/llm: ring-buffer call logger with stats
- Gateway: /api/v1/admin/llm-calls, /system, /plugins, /sessions endpoints
- Frontend: LLMCallsPage (table + stats), SystemPage (version/plugins/sessions)
- Admin nav expanded to 7 items

frontend/web/src/App.tsx

@@ -8,6 +8,8 @@ import ServicesPage from './pages/admin/ServicesPage'
 import IoTManagementPage from './pages/admin/IoTManagementPage'
 import PluginsPage from './pages/admin/PluginsPage'
 import MemoryPage from './pages/admin/MemoryPage'
+import LLMCallsPage from './pages/admin/LLMCallsPage'
+import SystemPage from './pages/admin/SystemPage'
 import LoginPage from './pages/LoginPage'
 
 export default function App() {
@@ -35,6 +37,8 @@ export default function App() {
           <Route path="iot" element={<IoTManagementPage />} />
           <Route path="plugins" element={<PluginsPage />} />
           <Route path="memory" element={<MemoryPage />} />
+          <Route path="llm-calls" element={<LLMCallsPage />} />
+          <Route path="system" element={<SystemPage />} />
         </Route>
       </Routes>
     </AppLayout>