fix(desktop): DeerFlow UI — ChatArea refactor + ai-elements + dead CSS cleanup

ChatArea retry button uses setInput instead of direct sendToGateway,
fix bootstrap spinner stuck for non-logged-in users,
remove dead CSS (aurora-title/sidebar-open/quick-action-chips),
add ai components (ReasoningBlock/StreamingText/ChatMode/ModelSelector/TaskProgress),
add ClassroomPlayer + ResizableChatLayout + artifact panel

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

desktop/src/hooks/useClassroom.ts

@@ -0,0 +1,76 @@
+/**
+ * useClassroom — React hook wrapping the classroom store for component consumption.
+ *
+ * Provides a simplified interface for classroom generation and chat.
+ */
+
+import { useCallback } from 'react';
+import {
+  useClassroomStore,
+  type GenerationRequest,
+} from '../store/classroomStore';
+import type {
+  Classroom,
+  ClassroomChatMessage,
+} from '../types/classroom';
+
+export interface UseClassroomReturn {
+  /** Is generation in progress */
+  generating: boolean;
+  /** Current generation stage name */
+  progressStage: string | null;
+  /** Progress percentage 0-100 */
+  progressPercent: number;
+  /** The active classroom */
+  activeClassroom: Classroom | null;
+  /** Chat messages for active classroom */
+  chatMessages: ClassroomChatMessage[];
+  /** Is a chat request loading */
+  chatLoading: boolean;
+  /** Error message, if any */
+  error: string | null;
+  /** Start classroom generation */
+  startGeneration: (request: GenerationRequest) => Promise<string>;
+  /** Cancel active generation */
+  cancelGeneration: () => void;
+  /** Send a chat message in the active classroom */
+  sendChatMessage: (message: string, sceneContext?: string) => Promise<void>;
+  /** Clear current error */
+  clearError: () => void;
+}
+
+/**
+ * Hook for classroom generation and multi-agent chat.
+ *
+ * Components should use this hook rather than accessing the store directly,
+ * to keep the rendering logic decoupled from state management.
+ */
+export function useClassroom(): UseClassroomReturn {
+  const {
+    generating,
+    progressStage,
+    progressPercent,
+    activeClassroom,
+    chatMessages,
+    chatLoading,
+    error,
+    startGeneration,
+    cancelGeneration,
+    sendChatMessage,
+    clearError,
+  } = useClassroomStore();
+
+  return {
+    generating,
+    progressStage,
+    progressPercent,
+    activeClassroom,
+    chatMessages,
+    chatLoading,
+    error,
+    startGeneration: useCallback((req: GenerationRequest) => startGeneration(req), [startGeneration]),
+    cancelGeneration: useCallback(() => cancelGeneration(), [cancelGeneration]),
+    sendChatMessage: useCallback((msg, ctx) => sendChatMessage(msg, ctx), [sendChatMessage]),
+    clearError: useCallback(() => clearError(), [clearError]),
+  };
+}