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/Util/WebinterfaceUtil.cs

@@ -1,19 +0,0 @@
-namespace ChatTwo.Util;
-
-public static class WebinterfaceUtil
-{
-    private static readonly Random Rng = new();
-
-    public static string GenerateSimpleAuthCode()
-    {
-        return (100000 + Rng.Next() % 100000).ToString()[1..];
-    }
-
-    public static string GenerateSimpleToken()
-    {
-        var buffer = new byte[15];
-        Rng.NextBytes(buffer);
-
-        return Convert.ToHexString(buffer);
-    }
-}