Introduce a bridge helper to get icon asset urls

app/helpers/bridge_helper.rb

@@ -0,0 +1,5 @@
+module BridgeHelper
+  def bridge_icon(name, extension = "svg")
+    asset_url("#{name}.#{extension}")
+  end
+end

app/views/columns/show/_add_card_button.html.erb

@@ -1,7 +1,7 @@
 <div class="board-tools card">
   <%= button_to board_cards_path(board), method: :post, class: "btn btn--link",
         form: { data: { turbo_frame: "_top" } },
-        data: { controller: "hotkey", action: "keydown.c@document->hotkey#click", bridge__buttons_target: "button", bridge_icon_name: "plus", bridge_title: "Add card" } do %>
+        data: { controller: "hotkey", action: "keydown.c@document->hotkey#click", bridge__buttons_target: "button", bridge_title: "Add card", bridge_icon_url: bridge_icon("add") } do %>
     <%= icon_tag "add", class: "show-on-touch" %>
     <span>Add a card</span>
     <kbd class="hide-on-touch">C</kbd>