feat: add custom Pivoine Rose theme to asciinema

- Mount custom CSS, favicon, and JS into static directories
- Create custom.js to inject theme CSS and favicon via JavaScript
- Add CustomThemeInjector Plug in custom.exs to inject script tag
- Custom theme features:
  - Pivoine rose primary color (#CE275B)
  - Gray tone backgrounds
  - Custom SVG favicon with rose gradient
  - Bootstrap 4 component overrides

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

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

asciinema/custom.exs

@@ -15,3 +15,43 @@ 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]