Topbar: add Custom Nodes Manager button (#7400)

Adds a desktop-only "Custom Nodes Manager" topbar button (Lucide puzzle)
in its own bordered island left of the actionbar. Button opens the
manager via useManagerState().openManager().

- New i18n key: menu.customNodesManager

<img width="318" height="147" alt="image"
src="https://github.com/user-attachments/assets/e36c5c7f-80d1-454c-87de-e0daa822fad1"
/>

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-7400-Topbar-add-Custom-Nodes-Manager-button-2c76d73d3650810b9122da61a3c4be39)
by [Unito](https://www.unito.io)

src/locales/en/main.json

@@ -807,6 +807,7 @@
     "dark": "Dark",
     "light": "Light",
     "manageExtensions": "Manage Extensions",
+    "customNodesManager": "Custom Nodes Manager",
     "settings": "Settings",
     "help": "Help",
     "queue": "Queue Panel"