Ship the first pluggable widget with a real frontend bundle (a search bar
for DuckDuckGo/Google/Bing + admin-managed custom engines), which required
widening widgetOrder from a closed 4-key union to a plain string[] so an
uploaded widget can be reordered relative to the built-ins with the same
up/down arrows, rather than always rendering after them. Also documents the
/widget-assets Nginx Proxy Manager location that a widget's frontendEntry
bundle needs to load at all — missing from the reverse-proxy docs since
nothing with a real frontendEntry existed until now.
