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/components/layout/Header.tsx

@@ -1,8 +1,10 @@
 /**
  * Header Component
- * Top navigation bar with page title
+ * Top navigation bar with page title and page-specific controls.
+ * On the Chat page, shows session selector, refresh, thinking toggle, and new session.
  */
 import { useLocation } from 'react-router-dom';
+import { ChatToolbar } from '@/pages/Chat/ChatToolbar';
 
 // Page titles mapping
 const pageTitles: Record<string, string> = {
@@ -16,13 +18,15 @@ const pageTitles: Record<string, string> = {
 
 export function Header() {
   const location = useLocation();
-  
-  // Get current page title
   const currentTitle = pageTitles[location.pathname] || 'ClawX';
-  
+  const isChatPage = location.pathname === '/chat';
+
   return (
-    <header className="flex h-14 items-center border-b bg-background px-6">
+    <header className="flex h-14 items-center justify-between border-b bg-background px-6">
       <h2 className="text-lg font-semibold">{currentTitle}</h2>
+      
+      {/* Chat-specific controls */}
+      {isChatPage && <ChatToolbar />}
     </header>
   );
 }