feat(ui/agents): add Composer

Textarea + Send at the bottom of the conversation pane. POSTs to
/v2/sessions/{sid}/followup; the resulting user_message lands via the
SSE stream.

ui/litellm-dashboard/src/app/(dashboard)/agents/components/three-pane/composer.tsx

@@ -0,0 +1,69 @@
+"use client";
+
+/**
+ * Composer — textarea + send button at the bottom of the conversation pane.
+ *
+ * On submit, POSTs to the followup endpoint and lets the SSE stream
+ * surface the resulting `user_message` event via the parent's event list.
+ */
+import { useState } from "react";
+import { Button, Input, message } from "antd";
+import { sendSessionFollowup } from "@/lib/cloud-agents-client";
+
+interface ComposerProps {
+  sessionId: string;
+  accessToken: string | null;
+  onSent?: () => void;
+}
+
+export default function Composer({ sessionId, accessToken, onSent }: ComposerProps) {
+  const [value, setValue] = useState("");
+  const [sending, setSending] = useState(false);
+
+  const handleSend = async () => {
+    const trimmed = value.trim();
+    if (!trimmed) return;
+    setSending(true);
+    try {
+      await sendSessionFollowup(accessToken, sessionId, trimmed);
+      setValue("");
+      onSent?.();
+    } catch (e) {
+      const errMsg = e instanceof Error ? e.message : String(e);
+      message.error(`Send failed: ${errMsg}`);
+    } finally {
+      setSending(false);
+    }
+  };
+
+  return (
+    <div className="border-t border-gray-200 p-3" data-testid="composer">
+      <div className="flex items-end gap-2">
+        <Input.TextArea
+          rows={2}
+          autoSize={{ minRows: 2, maxRows: 6 }}
+          value={value}
+          onChange={(e) => setValue(e.target.value)}
+          placeholder="Add a follow-up..."
+          disabled={sending}
+          data-testid="composer-input"
+          onPressEnter={(e) => {
+            if (!e.shiftKey) {
+              e.preventDefault();
+              void handleSend();
+            }
+          }}
+        />
+        <Button
+          type="primary"
+          loading={sending}
+          onClick={() => void handleSend()}
+          data-testid="composer-send"
+          disabled={!value.trim()}
+        >
+          Send
+        </Button>
+      </div>
+    </div>
+  );
+}