feat(web): add multi-select batch delete and undo flow

- add message selection mode from context menu

- support batch delete for me and conditional batch delete for everyone

- add undo snackbar for delete-for-me with delayed backend commit

- add restoreMessages helper in chat store for undo rollback

web/src/store/chatStore.ts

@@ -24,6 +24,7 @@ interface ChatState {
   removeOptimisticMessage: (chatId: number, clientMessageId: string) => void;
   setMessageDeliveryStatus: (chatId: number, messageId: number, status: DeliveryStatus) => void;
   removeMessage: (chatId: number, messageId: number) => void;
+  restoreMessages: (chatId: number, messages: Message[]) => void;
   clearChatMessages: (chatId: number) => void;
   incrementUnread: (chatId: number) => void;
   clearUnread: (chatId: number) => void;
@@ -158,6 +159,26 @@ export const useChatStore = create<ChatState>((set, get) => ({
       }
     }));
   },
+  restoreMessages: (chatId, messages) => {
+    if (!messages.length) {
+      return;
+    }
+    const old = get().messagesByChat[chatId] ?? [];
+    const byId = new Map<number, Message>();
+    for (const message of old) {
+      byId.set(message.id, message);
+    }
+    for (const message of messages) {
+      byId.set(message.id, message);
+    }
+    const merged = [...byId.values()].sort((a, b) => a.id - b.id);
+    set((state) => ({
+      messagesByChat: {
+        ...state.messagesByChat,
+        [chatId]: merged
+      }
+    }));
+  },
   clearChatMessages: (chatId) =>
     set((state) => ({
       messagesByChat: {