Fix AI chat: disable send without session

- Disable send button when no active session exists

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

src/app/ai/page.tsx

@@ -161,7 +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 text-white rounded-lg ${loading ? "bg-gray-300 cursor-not-allowed" : "bg-rose-400"}`}>
+            <button onClick={handleSend} disabled={loading || !currentSession} className={`px-4 py-2 text-white rounded-lg ${loading || !currentSession ? "bg-gray-300 cursor-not-allowed" : "bg-rose-400"}`}>
               {loading ? "..." : "Send"}
             </button>
           </div>