The Usage page auto-adds every noAuth free provider so connectionless
providers (opencode) still appear. It did not filter the registry's
hidden flag, so devin-cli and mimo-free — both category:"free" with
noAuth:true and hidden:true — showed up in Usage despite having no
connection and being absent from the Providers page (which does filter
hidden).
Extract the list assembly into buildUsageProviderList (shared/utils/
usageProviders.js) and skip hidden free providers there. Behavior for
visible noAuth providers (opencode) and dedup of active connections is
unchanged; covered by tests/unit/usage-provider-list.test.js.
