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/components/ai/ResizableChatLayout.tsx

@@ -62,7 +62,7 @@ export function ResizableChatLayout({
 
   if (!rightPanelOpen || !rightPanel) {
     return (
-      <div className="flex-1 flex flex-col overflow-hidden relative">
+      <div className="h-full flex flex-col overflow-hidden relative">
         {chatPanel}
         <button
           onClick={handleToggle}
@@ -76,7 +76,7 @@ export function ResizableChatLayout({
   }
 
   return (
-    <div className="flex-1 flex flex-col overflow-hidden">
+    <div className="h-full flex flex-col overflow-hidden">
       <Group
         orientation="horizontal"
         onLayoutChanged={(layout) => savePanelSizes(layout)}