Add mobile vault placeholder readiness model

[El-swaggerito]

Description

Difficulty: Hard

Type: Feature

Summary

Add a readiness model for the mobile vault placeholder.

Current Behaviour

The vault screen may show placeholder content without clearly explaining unavailable, planned, disabled, or ready states.

Expected Behaviour

The app should represent vault readiness states clearly and avoid implying unsupported functionality is live.

Suggested Implementation

Create readiness types, UI states, tests or fixtures, and documentation.

Files or Areas Likely Affected

• src/vault/
• src/screens/
• src/components/
• tests/
• docs/

Acceptance Criteria

• Vault readiness model is added.
• Unavailable, planned, disabled, and ready states are represented.
• UI copy avoids overpromising.
• Tests or fixtures cover readiness states.
• Docs explain current vault limitations.
• README references vault status where appropriate.

Additional Notes

This keeps contributor work honest and clear.