Fix XHS publish button detection

[walker1211]

Summary

• Click the current XHS custom publish element before falling back to the legacy button text selector.
• Keep disabled custom publish elements ignored.
• Add browser-backed coverage for the new selector path.

Test plan

• go test ./internal/xhs
• go test ./...
• MARK2NOTE_FULL_TESTS=1 go test ./internal/xhs -run 'TestClickOnlySelfPublishButton'