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/MessageManager.cs

@@ -258,19 +258,13 @@ internal class MessageManager : IAsyncDisposable
         if (Plugin.Config.DatabaseBattleMessages || !message.Code.IsBattle())
             Store.UpsertMessage(message);
 
-        var currentTabId = Plugin.CurrentTab.Identifier;
         var currentMatches = Plugin.CurrentTab.Matches(message);
         foreach (var tab in Plugin.Config.Tabs)
         {
             var unread = !(tab.UnreadMode == UnreadMode.Unseen && Plugin.CurrentTab != tab && currentMatches);
 
             if (tab.Matches(message))
-            {
                 tab.AddMessage(message, unread);
-
-                if (tab.Identifier == currentTabId)
-                    Plugin.ServerCore.SendNewMessage(message);
-            }
         }
     }