fix: remove broken CustomThemeInjector Plug from custom.exs

The Plug was causing AsciinemaWeb.Endpoint to fail on startup.
Reverting to just SMTP configuration while we find a proper way
to inject custom theme.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

asciinema/custom.exs

@@ -15,43 +15,3 @@ config :asciinema, Asciinema.Emails.Mailer,
     verify: :verify_none,
     versions: [:"tlsv1.2", :"tlsv1.3"]
   ]
-
-# Custom Plug to inject theme JavaScript into HTML responses
-defmodule CustomThemeInjector do
-  @behaviour Plug
-
-  def init(opts), do: opts
-
-  def call(conn, _opts) do
-    Plug.Conn.register_before_send(conn, fn conn ->
-      if html_response?(conn) do
-        inject_custom_script(conn)
-      else
-        conn
-      end
-    end)
-  end
-
-  defp html_response?(conn) do
-    case Plug.Conn.get_resp_header(conn, "content-type") do
-      [content_type | _] -> String.contains?(content_type, "text/html")
-      [] -> false
-    end
-  end
-
-  defp inject_custom_script(conn) do
-    {status, headers, body} = Plug.Conn.sent_resp(conn)
-
-    if is_binary(body) and String.contains?(body, "</head>") do
-      script_tag = ~s(<script src="/js/custom.js"></script>\n)
-      modified_body = String.replace(body, "</head>", "#{script_tag}</head>", global: false)
-      %{conn | status: status, resp_headers: headers, resp_body: modified_body}
-    else
-      conn
-    end
-  end
-end
-
-#  Add the custom plug to the endpoint
-config :asciinema, AsciinemaWeb.Endpoint,
-  http: [plug: CustomThemeInjector]