Prevent accidental gestures with a session control lock

[PurpleSentinel]

Parent: #23

Summary

Add an optional lock or deliberate confirmation pattern for session-ending gestures in a moving vehicle.

Acceptance criteria

• Allow the lock behavior to be enabled or disabled.
• Use a clearly documented gesture that remains achievable with gloves.
• Provide immediate visible feedback when an action is blocked or awaiting confirmation.
• Retain an accessible emergency exit path.
• Test debounce, timeout, cancel, and confirmed action paths.

Roadmap group

Driver display and alerts