Add secure session storage and biometric lock option for mobile

[inteee]

Description

For a healthcare-facing mobile app, even an MVP benefits from better session handling and quick relock behavior.

Scope

• Store tokens in secure storage
• Add optional app relock on resume timeout
• If platform support is easy, add biometric unlock toggle
• Keep fallback PIN-less if time is limited

Acceptance Criteria

• Mobile session tokens are not stored in plain async storage
• App can relock after inactivity
• Feature is optional and documented if biometric support is partial

Dependencies

Blocked by #220

Stack

React Native