[Release] Validate the MVP on iPhone, MacBook, and offline

[troyscott]

Outcome

Collect final evidence that the MVP works in the real study workflow.

Acceptance criteria

• iPhone Safari and installed Home Screen app are exercised.
• MacBook Safari plus Chromium and Firefox are exercised.
• Phone, tablet, and desktop layouts are visually reviewed.
• Offline synchronized-chapter behavior is verified.
• Progress and annotations survive a content publication.
• Release notes state verified behavior and known limitations.