Improve Dataset Viewer accessible interactions #1636

Description

Improve Dataset Viewer keyboard, pointer, status, language, and generated API documentation behavior for accessible annotation workflows. Add exact semantic browser coverage so each high-risk Viewer state has an owned verification case.

Acceptance Criteria

• Joint and group toggles expose synchronized pressed state and native keyboard activation.
• Graph selection cancellation preserves the prior playback state and handles unmount and duplicate terminal events.
• Language tags use one tested validation corpus and invalid values cannot be saved.
• Save, completion, and failure messages use stable status or alert regions without stealing focus.
• Swagger and ReDoc pages remain usable at desktop and narrow viewport widths.
• The semantic Playwright manifest closes every declared Viewer accessibility case exactly once.

Related

• Evidence source: comprehensive accessibility review
• Scope: Dataset Viewer interaction and browser evidence surfaces