fix(ui): panel toggle in header bar + message spacing

- Move side panel toggle from floating button to chat header right side
  (Trae Solo style) via new PanelToggleButton component
- Add px-6 py-4 padding to message list container
- Add mb-5 gap between messages for readable vertical spacing

desktop/src/components/ChatArea.tsx

@@ -12,7 +12,7 @@ import { type UnlistenFn } from '@tauri-apps/api/event';
 import { safeListenEvent } from '../lib/safe-tauri';
 import { Paperclip, ArrowUp, MessageSquare, Download, X, FileText, Image as ImageIcon, Search, ClipboardList, Square } from 'lucide-react';
 import { Button, EmptyState, MessageListSkeleton, LoadingDots } from './ui';
-import { ResizableChatLayout } from './ai/ResizableChatLayout';
+import { ResizableChatLayout, PanelToggleButton } from './ai/ResizableChatLayout';
 import { ArtifactPanel } from './ai/ArtifactPanel';
 import { ToolCallChain, type ToolCallStep } from './ai/ToolCallChain';
 import { TaskProgress, type Subtask } from './ai/TaskProgress';
@@ -361,6 +361,13 @@ export function ChatArea({ compact, onOpenDetail }: { compact?: boolean; onOpenD
               <Search className="w-3.5 h-3.5" />
             </Button>
           )}
+          {/* Side panel toggle — Trae Solo style, in header */}
+          {!compact && (
+            <PanelToggleButton
+              panelOpen={artifactPanelOpen}
+              onToggle={() => setArtifactPanelOpen(!artifactPanelOpen)}
+            />
+          )}
           {/* 详情按钮 (简洁模式) */}
           {compact && onOpenDetail && (
             <Button
@@ -398,7 +405,7 @@ export function ChatArea({ compact, onOpenDetail }: { compact?: boolean; onOpenD
       </AnimatePresence>
 
       {/* Messages */}
-      <Conversation className="flex-1 bg-white dark:bg-gray-900">
+      <Conversation className="flex-1 bg-white dark:bg-gray-900 px-6 py-4">
         <AnimatePresence mode="popLayout">
           {/* Loading skeleton */}
           {isLoading && messages.length === 0 && (
@@ -467,6 +474,7 @@ export function ChatArea({ compact, onOpenDetail }: { compact?: boolean; onOpenD
                 animate="visible"
                 layout
                 transition={defaultTransition}
+                className="mb-5"
               >
                 <MessageBubble message={message} onRetry={createRetryHandler(message.id)} />
               </motion.div>