feat(web): per-chat message drafts

- store unsent draft text per chat in zustand
- restore draft when switching chats
- clear draft after successful send

web/src/components/MessageComposer.tsx

@@ -7,6 +7,9 @@ import { buildWsUrl } from "../utils/ws";
 export function MessageComposer() {
   const activeChatId = useChatStore((s) => s.activeChatId);
   const me = useAuthStore((s) => s.me);
+  const draftsByChat = useChatStore((s) => s.draftsByChat);
+  const setDraft = useChatStore((s) => s.setDraft);
+  const clearDraft = useChatStore((s) => s.clearDraft);
   const addOptimisticMessage = useChatStore((s) => s.addOptimisticMessage);
   const confirmMessageByClientId = useChatStore((s) => s.confirmMessageByClientId);
   const removeOptimisticMessage = useChatStore((s) => s.removeOptimisticMessage);
@@ -25,6 +28,19 @@ export function MessageComposer() {
   const [previewUrl, setPreviewUrl] = useState<string | null>(null);
   const [isRecording, setIsRecording] = useState(false);
 
+  useEffect(() => {
+    if (!activeChatId) {
+      if (text !== "") {
+        setText("");
+      }
+      return;
+    }
+    const draft = draftsByChat[activeChatId] ?? "";
+    if (draft !== text) {
+      setText(draft);
+    }
+  }, [activeChatId, draftsByChat, text]);
+
   useEffect(() => {
     return () => {
       if (previewUrl) {
@@ -64,6 +80,7 @@ export function MessageComposer() {
       const message = await sendMessageWithClientId(activeChatId, textValue, "text", clientMessageId, replyToMessageId);
       confirmMessageByClientId(activeChatId, clientMessageId, message);
       setText("");
+      clearDraft(activeChatId);
       setReplyToMessage(activeChatId, null);
       const ws = getWs();
       ws?.send(JSON.stringify({ event: "typing_stop", payload: { chat_id: activeChatId } }));
@@ -284,7 +301,11 @@ export function MessageComposer() {
           placeholder="Write a message..."
           value={text}
           onChange={(e) => {
-            setText(e.target.value);
+            const next = e.target.value;
+            setText(next);
+            if (activeChatId) {
+              setDraft(activeChatId, next);
+            }
             if (activeChatId) {
               const ws = getWs();
               ws?.send(JSON.stringify({ event: "typing_start", payload: { chat_id: activeChatId } }));