fix: ensure last active community is reset when leaving community

There's a bug where the last active community will be set in local settings
and not removed when a user leaves a community.

This causes the app to start on an invalid screen/state as it still tries to
render the community view for a community that the user has just left.

This commit ensure we're resetting the last active community in the local settings
when a users leaves the community.

Fixes: #4094

ui/app/AppLayouts/Chat/stores/RootStore.qml

@@ -136,6 +136,9 @@ QtObject {
 
     function leaveCommunity(communityId) {
         chatsModelInst.communities.leaveCommunity(communityId);
+        if (communityId == chatsModelInst.communities.activeCommunity.id) {
+            localAccountSensitiveSettings.lastModeActiveCommunity = ""
+        }
     }
 
     function setCommunityMuted(communityId, checked) {