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/components/composer.test.tsx

@@ -176,6 +176,7 @@ vi.mock("lucide-react-native", () => {
   const createIcon = (name: string) => (props: Record<string, unknown>) =>
     React.createElement("span", { ...props, "data-icon": name });
   return {
+    ArrowUp: createIcon("ArrowUp"),
     CornerDownLeft: createIcon("CornerDownLeft"),
     Square: createIcon("Square"),
     Pencil: createIcon("Pencil"),