Add Tests iOS/Screens/ page objects and fix gitignore

The screens/ gitignore rule was matching Tests iOS/Screens/ on
case-insensitive macOS. Anchored to /screens/ (repo root only) so
the 7 UI test page object files are no longer ignored.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -76,5 +76,5 @@ Secrets.swift
 **/Secrets.swift
 
 # Screenshots and promo assets
-screens/
+/screens/
 feels-promo/