feat(drafts): persist chat drafts in localStorage

- load drafts from localStorage on startup
- write drafts to localStorage on update/clear
- keep per-chat draft restore across page reload

web/src/store/chatStore.ts

@@ -2,6 +2,42 @@ import { create } from "zustand";
 import { getChats, getMessages, updateMessageStatus } from "../api/chats";
 import type { Chat, DeliveryStatus, Message, MessageType } from "../chat/types";
 
+const DRAFTS_STORAGE_KEY = "bm_drafts_v1";
+
+function loadDraftsFromStorage(): Record<number, string> {
+  if (typeof window === "undefined") {
+    return {};
+  }
+  try {
+    const raw = window.localStorage.getItem(DRAFTS_STORAGE_KEY);
+    if (!raw) {
+      return {};
+    }
+    const parsed = JSON.parse(raw) as Record<string, string>;
+    const result: Record<number, string> = {};
+    for (const [key, value] of Object.entries(parsed)) {
+      const chatId = Number(key);
+      if (Number.isFinite(chatId) && typeof value === "string") {
+        result[chatId] = value;
+      }
+    }
+    return result;
+  } catch {
+    return {};
+  }
+}
+
+function saveDraftsToStorage(drafts: Record<number, string>): void {
+  if (typeof window === "undefined") {
+    return;
+  }
+  try {
+    window.localStorage.setItem(DRAFTS_STORAGE_KEY, JSON.stringify(drafts));
+  } catch {
+    return;
+  }
+}
+
 interface ChatState {
   chats: Chat[];
   activeChatId: number | null;
@@ -52,7 +88,7 @@ export const useChatStore = create<ChatState>((set, get) => ({
   chats: [],
   activeChatId: null,
   messagesByChat: {},
-  draftsByChat: {},
+  draftsByChat: loadDraftsFromStorage(),
   hasMoreByChat: {},
   loadingMoreByChat: {},
   typingByChat: {},
@@ -331,12 +367,14 @@ export const useChatStore = create<ChatState>((set, get) => ({
       })
     })),
   setDraft: (chatId, text) =>
-    set((state) => ({
-      draftsByChat: {
+    set((state) => {
+      const nextDrafts = {
         ...state.draftsByChat,
         [chatId]: text
-      }
-    })),
+      };
+      saveDraftsToStorage(nextDrafts);
+      return { draftsByChat: nextDrafts };
+    }),
   clearDraft: (chatId) =>
     set((state) => {
       if (!(chatId in state.draftsByChat)) {
@@ -344,6 +382,7 @@ export const useChatStore = create<ChatState>((set, get) => ({
       }
       const next = { ...state.draftsByChat };
       delete next[chatId];
+      saveDraftsToStorage(next);
       return { draftsByChat: next };
     }),
   setFocusedMessage: (chatId, messageId) =>