chore: stop tracking Xcode xcuserstate (UI-state noise)

The existing ignore rules were anchored to repo root, so the nested
*.xcodeproj/project.xcworkspace/xcuserdata/ never matched and kept
showing up as a change on every commit. Add **/xcuserdata/ and
*.xcuserstate and untrack the file.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitignore

@@ -1,4 +1,6 @@
 # Xcode
+**/xcuserdata/
+*.xcuserstate
 *.xcodeproj/xcuserdata/
 *.xcworkspace/xcuserdata/
 *.xcworkspace/contents.xcworkspacedata