fix: thread allowEmptySubmit through submit pipeline

submitAgentInput returned "noop" for empty messages regardless of
allowEmptySubmit, breaking the new-workspace empty-submit flow.
Now the flag is passed from Composer through to submitAgentInput
so empty submissions are allowed when the prop is set.

packages/app/src/components/composer.tsx

@@ -354,6 +354,7 @@ export function Composer({
     await submitAgentInput({
       message,
       imageAttachments,
+      allowEmptySubmit,
       forceSend,
       isAgentRunning: agentState.status === "running",
       // Parent-managed submits are still valid submit paths even when the