Remove webinterface server, HTTP routes and Svelte frontend

Drops the entire ChatTwo/Http/ tree (ServerCore, HostContext,
RouteController, Processing, SSEConnection, the message protocol DTOs
and the bundled Svelte frontend) plus WebinterfaceUtil. Also removes
every ServerCore.Send* call site that fed the SSE stream:

  - MessageManager.ProcessMessage no longer broadcasts new messages
  - Chat.cs no longer notifies on login
  - PayloadHandler no longer rebroadcasts on screenshot-mode toggle
  - ChatLogWindow no longer announces tab and channel switches

The Plugin class drops the ServerCore field, the auto-start branch and
the Dispose hook. The DbViewer still imported a stale namespace from
the message protocol; the using is removed.

Language.resx and its generated Designer file keep the Webinterface
string keys for now so future upstream cherry-picks do not break on
missing resources. They are dead code from our perspective but harmless.

ChatTwo/Ui/ChatLogWindow.cs

@@ -375,10 +375,6 @@ public sealed class ChatLogWindow : Window
             newTab.CurrentChannel = previousTab.CurrentChannel;
 
         SetChannel(newTab.CurrentChannel.Channel);
-
-        // Inform the webinterface about tab switch
-        // TODO implement tabs in the webinterface
-        Plugin.ServerCore.SendNewLogin();
     }
 
     private enum HideState
@@ -772,10 +768,7 @@ public sealed class ChatLogWindow : Window
     {
         var currentChannel = ReadChannelName(activeTab);
         if (!currentChannel.SequenceEqual(PreviousChannel))
-        {
             PreviousChannel = currentChannel;
-            Plugin.ServerCore.SendChannelSwitch(currentChannel);
-        }
 
         DrawChunks(currentChannel);
     }