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/store/chatStore.ts

@@ -6,6 +6,7 @@ interface ChatState {
   chats: Chat[];
   activeChatId: number | null;
   messagesByChat: Record<number, Message[]>;
+  draftsByChat: Record<number, string>;
   typingByChat: Record<number, number[]>;
   replyToByChat: Record<number, Message | null>;
   unreadBoundaryByChat: Record<number, number>;
@@ -38,12 +39,15 @@ interface ChatState {
   setReplyToMessage: (chatId: number, message: Message | null) => void;
   updateChatPinnedMessage: (chatId: number, pinnedMessageId: number | null) => void;
   applyPresenceEvent: (chatId: number, userId: number, isOnline: boolean, lastSeenAt?: string) => void;
+  setDraft: (chatId: number, text: string) => void;
+  clearDraft: (chatId: number) => void;
 }
 
 export const useChatStore = create<ChatState>((set, get) => ({
   chats: [],
   activeChatId: null,
   messagesByChat: {},
+  draftsByChat: {},
   typingByChat: {},
   replyToByChat: {},
   unreadBoundaryByChat: {},
@@ -272,5 +276,21 @@ export const useChatStore = create<ChatState>((set, get) => ({
         }
         return chat;
       })
-    }))
+    })),
+  setDraft: (chatId, text) =>
+    set((state) => ({
+      draftsByChat: {
+        ...state.draftsByChat,
+        [chatId]: text
+      }
+    })),
+  clearDraft: (chatId) =>
+    set((state) => {
+      if (!(chatId in state.draftsByChat)) {
+        return state;
+      }
+      const next = { ...state.draftsByChat };
+      delete next[chatId];
+      return { draftsByChat: next };
+    })
 }));