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/index.css

@@ -1,27 +1,5 @@
 @import "tailwindcss";
 
-/* Aurora gradient animation for welcome title (DeerFlow-inspired) */
-@keyframes gradient-shift {
-  0%, 100% { background-position: 0% 50%; }
-  50% { background-position: 100% 50%; }
-}
-
-.aurora-title {
-  background: linear-gradient(
-    135deg,
-    #f97316 0%,     /* orange-500 */
-    #ef4444 25%,    /* red-500 */
-    #f97316 50%,    /* orange-500 */
-    #fb923c 75%,    /* orange-400 */
-    #f97316 100%    /* orange-500 */
-  );
-  background-size: 200% 200%;
-  -webkit-background-clip: text;
-  background-clip: text;
-  -webkit-text-fill-color: transparent;
-  animation: gradient-shift 4s ease infinite;
-}
-
 :root {
   /* Brand Colors - 中性灰色系 */
   --color-primary: #374151;           /* gray-700 */
@@ -154,3 +132,38 @@ textarea:focus-visible {
   outline: none !important;
   box-shadow: none !important;
 }
+
+/* === Accessibility: reduced motion === */
+@media (prefers-reduced-motion: reduce) {
+  *, *::before, *::after {
+    animation-duration: 0.01ms !important;
+    animation-iteration-count: 1 !important;
+    transition-duration: 0.01ms !important;
+    scroll-behavior: auto !important;
+  }
+}
+
+/* === Responsive breakpoints for small windows/tablets === */
+@media (max-width: 768px) {
+  /* Auto-collapse sidebar aside on narrow viewports */
+  aside.w-64 {
+    width: 0 !important;
+    min-width: 0 !important;
+    overflow: hidden;
+    border-right: none !important;
+  }
+  aside.w-64.sidebar-open {
+    width: 260px !important;
+    min-width: 260px !important;
+    position: fixed;
+    z-index: 50;
+    height: 100vh;
+  }
+}
+
+@media (max-width: 480px) {
+  .chat-bubble-assistant,
+  .chat-bubble-user {
+    max-width: 95% !important;
+  }
+}