ListChannelReplies used getChannelForMemberOrLinkedGuest, so messages.getReplies
was member-only. ListChannelHistory (flat history) uses getChannelForViewer and
already allows a public channel's non-members to preview it. The mismatch meant
that on a public forum you could preview the flat history but not the topics -
and after leaving, tdesktop's topic view got CHANNEL_PRIVATE and sat on
"Loading..." forever instead of rendering a preview.
Switch the primary channel lookup in ListChannelReplies (both stores) to the
viewer-scope path. Private channels still return CHANNEL_PRIVATE to non-members;
the broadcast comment-thread lookup is unchanged.
ListForumTopics / GetForumTopicsByID / GeneralForumTopic gated on
membership while channel history uses the public-preview path, so a
public forum's topics (General included) were invisible until you joined.
Switch them to getChannelForViewer / channelForViewerLocked; private
forums and write paths keep the membership gate.
