feat(app): gate return-key submit icon to new workspace composer

The new-workspace composer is a one-shot "create" submit, not a chat send, so
a return-key glyph reads more accurately there than an arrow. Add a submitIcon
prop on Composer/MessageInput (defaulting to "arrow") and opt NewWorkspace
into "return".

packages/app/src/screens/new-workspace-screen.tsx

@@ -462,6 +462,7 @@ export function NewWorkspaceScreen({
             onSubmitMessage={handleCreateChatAgent}
             allowEmptySubmit={true}
             submitButtonAccessibilityLabel="Create"
+            submitIcon="return"
             isSubmitLoading={pendingAction === "chat"}
             submitBehavior="preserve-and-lock"
             blurOnSubmit={true}