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/lib/saas-relay.ts

@@ -55,6 +55,7 @@ export function installRelayMethods(ClientClass: { prototype: any }): void {
       _serverReachable: boolean;
       _isAuthEndpoint(path: string): boolean;
       refreshToken(): Promise<string>;
+      refreshMutex(): Promise<string>;
     },
     body: unknown,
     signal?: AbortSignal,
@@ -87,7 +88,7 @@ export function installRelayMethods(ClientClass: { prototype: any }): void {
         // On 401, attempt token refresh once
         if (response.status === 401 && attempt === 0 && !this._isAuthEndpoint('/api/v1/relay/chat/completions')) {
           try {
-            const newToken = await this.refreshToken();
+            const newToken = await this.refreshMutex();
             if (newToken) continue; // Retry with refreshed token
           } catch (e) {
             logger.debug('Token refresh failed', { error: e });