fix(chat): move toolbar to Header and add New Session button

- Move ChatToolbar (session selector, refresh, thinking toggle) from
  the Chat page body into the Header component, so controls appear
  at the same level as the "Chat" title
- Add New Session button (+) to create a fresh conversation
- Add newSession action to chat store
- Header conditionally renders ChatToolbar only on /chat route
- Chat page fills full content area without duplicate toolbar

src/pages/Chat/index.tsx

@@ -10,7 +10,6 @@ import { Card, CardContent } from '@/components/ui/card';
 import { useChatStore } from '@/stores/chat';
 import { useGatewayStore } from '@/stores/gateway';
 import { LoadingSpinner } from '@/components/common/LoadingSpinner';
-import { ChatToolbar } from './ChatToolbar';
 import { ChatMessage } from './ChatMessage';
 import { ChatInput } from './ChatInput';
 import { extractText } from './message-utils';
@@ -63,13 +62,7 @@ export function Chat() {
   const streamText = streamingMessage ? extractText(streamingMessage) : '';
 
   return (
-    <div className="flex h-[calc(100vh-4rem)] flex-col">
-      {/* Toolbar: session selector, refresh, thinking toggle */}
-      <div className="flex items-center justify-between px-4 py-2 border-b">
-        <div /> {/* spacer */}
-        <ChatToolbar />
-      </div>
-
+    <div className="flex flex-col -m-6" style={{ height: 'calc(100vh - 3.5rem)' }}>
       {/* Messages Area */}
       <div className="flex-1 overflow-y-auto px-4 py-4">
         <div className="max-w-4xl mx-auto space-y-4">