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/error-handling.ts

@@ -56,12 +56,19 @@ function initErrorStore(): void {
     errors: [],
 
     addError: (error: AppError) => {
+      // Dedup: skip if same title+message already exists and undismissed
+      const isDuplicate = errorStore.errors.some(
+        (e) => !e.dismissed && e.title === error.title && e.message === error.message
+      );
+      if (isDuplicate) return;
+
       const storedError: StoredError = {
         ...error,
         dismissed: false,
         reported: false,
       };
-      errorStore.errors = [storedError, ...errorStore.errors];
+      // Cap at 50 errors to prevent unbounded growth
+      errorStore.errors = [storedError, ...errorStore.errors].slice(0, 50);
       // Notify listeners
       notifyErrorListeners(error);
     },