refactor: repartition server-side and client-side code

app/.client/stores/chat-message.ts

@@ -0,0 +1,16 @@
+import { atom } from 'nanostores';
+import type { UPageMessageMetadata } from '~/types/message';
+
+export type SendChatMessageParams = {
+  messageContent: string;
+  files: File[];
+  metadata?: UPageMessageMetadata;
+};
+
+export type SendChatMessageFunction = (params: SendChatMessageParams) => Promise<void>;
+
+export const sendChatMessageStore = atom<SendChatMessageFunction | null>(null);
+
+export function setSendChatMessage(fn: SendChatMessageFunction) {
+  sendChatMessageStore.set(fn);
+}