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/GameFunctions/Chat.cs

@@ -156,9 +156,6 @@ internal sealed unsafe class Chat : IDisposable
             return;
 
         ChangeChannelNameDetour(agent);
-
-        // Inform all clients that a new login happened
-        Plugin.ServerCore.SendNewLogin();
     }
 
     private byte ChatLogRefreshDetour(nint log, ushort eventId, AtkValue* value)