Fix AI chat page: add back button, disable send while loading

- Add back link (←) to /menu in header
- Send button: gray out while loading to prevent double-click

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/app/ai/page.tsx

@@ -138,6 +138,7 @@ export default function AIChatPage() {
       {/* Main chat */}
       <div className="flex-1 flex flex-col">
         <div className="p-4 flex items-center border-b bg-white">
+          <a href="/menu" className="p-2">←</a>
           <button onClick={() => setSidebarOpen(!sidebarOpen)} className="p-2">☰</button>
           <span className="ml-2 font-medium">{currentSession?.title || "AI Chat"}</span>
         </div>
@@ -160,7 +161,7 @@ export default function AIChatPage() {
               className="flex-1 p-3 border rounded-lg"
               disabled={loading}
             />
-            <button onClick={handleSend} disabled={loading} className="px-4 py-2 bg-rose-400 text-white rounded-lg">
+            <button onClick={handleSend} disabled={loading} className={`px-4 py-2 text-white rounded-lg ${loading ? "bg-gray-300 cursor-not-allowed" : "bg-rose-400"}`}>
               {loading ? "..." : "Send"}
             </button>
           </div>