feat: Add a read indicator for web-widget channel (#4224)

Co-authored-by: Pranav Raj S <pranav@chatwoot.com>

app/javascript/dashboard/store/modules/conversations/index.js

@@ -13,6 +13,7 @@ const state = {
   currentInbox: null,
   selectedChatId: null,
   appliedFilters: [],
+  conversationLastSeen: null,
 };
 
 // mutations
@@ -33,6 +34,9 @@ export const mutations = {
     _state.allConversations = [];
     _state.selectedChatId = null;
   },
+  [types.SET_CONVERSATION_LAST_SEEN](_state, timestamp) {
+    _state.conversationLastSeen = timestamp;
+  },
   [types.SET_ALL_MESSAGES_LOADED](_state) {
     const [chat] = getSelectedChatConversation(_state);
     Vue.set(chat, 'allMessagesLoaded', true);