feat(chat): put the channel header above both windows

Drawn before the scroll child in each, so it stays put while the log moves. The
child's height is deliberately left alone: it is given as a negative value, and
ImGui resolves those against the space still available from the current cursor --
which the header has already reduced. Subtracting it a second time would have
opened a gap of exactly the header's height above the input row, in both windows.

Where the name shows follows one rule: never twice on the same screen. The
sidebar layout is the only place nothing else carries it, so that is the only
place the header says it. The top-tab strip carries it, and a pop-out with its
title bar on carries it in the title.

The pop-out's plain title row is gone, replaced by the header. The comment it
left behind is worth keeping in mind -- an earlier header row was removed exactly
because it repeated the tab name one line below the title bar. That warning is
now the rule rather than a reason to have no header at all.

The trailing detail is the home world and the clock, and the clock follows the
same Use24HourClock setting the message timestamps do. Two clock formats in one
window, with the header sitting directly above a column of timestamps, would be a
defect rather than a preference. Culture is pinned for the same reason it is
pinned in the message list.

HellionChat/Resources/HellionStrings.pl.resx

@@ -1357,4 +1357,7 @@
   <data name="Settings_Preview_StatusOpen" xml:space="preserve">
     <value>otwarte</value>
   </data>
+  <data name="ChannelHeader_NotLoggedIn" xml:space="preserve">
+    <value>Niezalogowany</value>
+  </data>
 </root>