Add a temporary touch interaction lock

[mokhov]

Problem

Holding or carrying a touch device can cause unwanted page turns and reader actions.

Proposed behavior

Allow readers to temporarily lock navigation gestures while keeping a clear, deliberate unlock path.

Acceptance criteria

• The locked state blocks page-turn taps and swipes without hiding the current page.
• A visible and screen-reader-accessible unlock control remains available.
• Unlocking requires a deliberate gesture that is unlikely to occur accidentally.
• The lock does not survive a browser reload or opening another book unless explicitly configured.
• Keyboard Escape and accessibility recovery paths are documented and tested.