fix: thread allowEmptySubmit through composer submit path

The Create button on the new workspace screen was visible but clicking
it did nothing — submitAgentInput() still rejected empty payloads as
"noop". Thread allowEmptySubmit through Composer into submitAgentInput
so the callback fires and workspace creation + redirect completes.

Adds regression test for the empty-submit path.

packages/app/src/components/composer.tsx

@@ -359,6 +359,7 @@ export function Composer({
       message,
       imageAttachments,
       hasExternalContent,
+      allowEmptySubmit,
       forceSend,
       isAgentRunning: agentState.status === "running",
       // Parent-managed submits are still valid submit paths even when the