Add UI smoke tests for Invidious behind HTTP Basic Auth

Three end-to-end specs that exercise the new basic-auth flows against a
real Invidious instance fronted by an nginx reverse proxy:

  1. add flow: types the URL, hits Detect, fills the basic-auth fields
     when the basicAuthRequired UI state appears, taps Retry Detection,
     and confirms the instance lands in the Sources list.

  2. state assertion: types a URL, taps Detect, and verifies the form
     transitions into the basicAuthRequired state (Retry Detection button
     present, no detected type) when no credentials were supplied.

  3. proxied login: ensures the instance exists, then drives the standard
     Invidious login flow with the proxied account credentials. Confirms
     the SID Cookie auth coexists with the per-client Authorization
     header on the basic-auth-aware HTTPClient.

Test infrastructure additions:

- spec/ui/support/config.rb: env-driven accessors for the basic-auth URL
  and proxied-account credentials. No secrets committed.

- spec/ui/support/instance_setup.rb: helpers
  add_invidious_with_basic_auth, remove_and_add_invidious_with_basic_auth,
  find_basic_auth_text_fields (mirroring find_auth_text_fields), and
  fill_field for tapping a discovered field by frame and typing into it.

All three specs skip cleanly when the relevant env vars are not set.

spec/ui/support/config.rb

@@ -112,6 +112,46 @@ module UITest
         invidious_email && invidious_password
       end
 
+      # Invidious URL for an instance fronted by HTTP Basic Auth (configurable via env)
+      def invidious_basic_auth_url
+        ENV.fetch('INVIDIOUS_BASIC_AUTH_URL', 'https://i03.s.yattee.stream')
+      end
+
+      # Extract host from the basic-auth Invidious URL
+      def invidious_basic_auth_host
+        URI.parse(invidious_basic_auth_url).host
+      end
+
+      # HTTP Basic Auth username for the proxy in front of the Invidious instance
+      def invidious_basic_auth_username
+        ENV.fetch('INVIDIOUS_BASIC_AUTH_USERNAME', nil)
+      end
+
+      # HTTP Basic Auth password for the proxy in front of the Invidious instance
+      def invidious_basic_auth_password
+        ENV.fetch('INVIDIOUS_BASIC_AUTH_PASSWORD', nil)
+      end
+
+      # Whether the basic-auth proxy credentials for Invidious are configured
+      def invidious_basic_auth_credentials?
+        invidious_basic_auth_username && invidious_basic_auth_password
+      end
+
+      # Invidious account email/username behind the basic-auth proxy
+      def invidious_proxied_email
+        ENV.fetch('INVIDIOUS_PROXIED_EMAIL', nil)
+      end
+
+      # Invidious account password behind the basic-auth proxy
+      def invidious_proxied_password
+        ENV.fetch('INVIDIOUS_PROXIED_PASSWORD', nil)
+      end
+
+      # Whether the proxied Invidious account credentials are configured
+      def invidious_proxied_credentials?
+        invidious_proxied_email && invidious_proxied_password
+      end
+
       # Piped URL for testing (configurable via env)
       def piped_url
         ENV.fetch('PIPED_URL', 'https://pipedapi.home.arekf.net')