feat(desktop): DeerFlow visual redesign + stream hang fix + intelligence client

DeerFlow frontend visual overhaul:
- Card-style input box (white rounded card, textarea top, actions bottom)
- Dropdown mode selector (闪速/思考/Pro/Ultra with icons+descriptions)
- Colored quick-action chips (小惊喜/写作/研究/收集/学习)
- Minimal top bar (title + token count + export)
- Warm gray color system (#faf9f6 bg, #f5f4f1 sidebar, #e8e6e1 border)
- DeerFlow-style sidebar (新对话/对话/智能体 nav)
- Reasoning block, tool call chain, task progress visualization
- Streaming text, model selector, suggestion chips components
- Resizable artifact panel with drag handle
- Virtualized message list for 100+ messages

Bug fixes:
- Stream hang: GatewayClient onclose code 1000 now calls onComplete
- WebView2 textarea border: CSS !important override for UA styles
- Gateway stream event handling (response/phase/tool_call types)

Intelligence client:
- Unified client with fallback drivers (compactor/heartbeat/identity/memory/reflection)
- Gateway API types and type conversions

desktop/src/index.css

@@ -1,5 +1,27 @@
 @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 */
@@ -18,8 +40,8 @@
 
   /* Neutral Colors */
   --color-bg: #ffffff;
-  --color-bg-secondary: #f9fafb;
-  --color-border: #e5e7eb;
+  --color-bg-secondary: #faf9f6;
+  --color-border: #e8e6e1;
   --color-text: #111827;
   --color-text-secondary: #6b7280;
   --color-text-muted: #9ca3af;
@@ -50,9 +72,9 @@
 }
 
 .dark {
-  --color-bg: #0f172a;
-  --color-bg-secondary: #1e293b;
-  --color-border: #334155;
+  --color-bg: #0f1117;
+  --color-bg-secondary: #1a1b26;
+  --color-border: #2e303a;
   --color-text: #f1f5f9;
   --color-text-secondary: #94a3b8;
   --color-text-muted: #64748b;
@@ -66,7 +88,7 @@
 
 body {
   font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
-  background-color: #f9fafb;
+  background-color: #faf9f6;
 }
 
 .custom-scrollbar::-webkit-scrollbar {
@@ -84,6 +106,14 @@ body {
   background: #4b5563;  /* gray-600 */
 }
 
+/* Sidebar warm background — DeerFlow-style */
+.sidebar-bg {
+  background: #f5f4f1;
+}
+:root.dark .sidebar-bg {
+  background: #0f1117;
+}
+
 .chat-bubble-assistant {
   background: white;
   border: 1px solid #e5e7eb;
@@ -111,3 +141,16 @@ body {
   0%, 100% { background-position: 0% 50%; }
   50% { background-position: 100% 50%; }
 }
+
+/* Force remove textarea border — WebView2 / Tailwind v4 preflight override */
+textarea {
+  border: none !important;
+  outline: none !important;
+  box-shadow: none !important;
+}
+textarea:focus,
+textarea:focus-visible {
+  border: none !important;
+  outline: none !important;
+  box-shadow: none !important;
+}