fix: apply code review fixes — double layout, SSE leaks, interval leak, conflict values

- App.tsx: remove double-wrapped MainLayout from /notifications route
- NotificationBell.tsx: decouple SSE from popover open state using ref
- useNotifications.ts: fix interval leak in onerror via ref in cleanup
- notifications.controller.ts: unsubscribe on req.on('close') for SSE Subject
- schedules.controller.ts: use existing schedule values in conflict catch

apps/admin/src/App.tsx

@@ -232,13 +232,7 @@ const App: React.FC = () => {
                 }
               />
 
-              <Route path="/notifications" element={
-                <PrivateRoute>
-                  <MainLayout />
-                </PrivateRoute>
-              }>
-                <Route index element={<NotificationsPage />} />
-              </Route>
+              <Route path="notifications" element={<NotificationsPage />} />
             </Route>
           </Routes>
         </BrowserRouter>