Client security: protect ZKP identity custody and block unsafe preview registration (TRR-19, TRR-20)

[NeaBouli]

Parent audit register: #303
Paused integration task: #300
Rollout tracker: #29

Objective

Prevent preview identities from leaking secrets or registering unusable commitments while the real browser prover remains non-submittable.

Acceptance criteria

• Do not persist a plaintext identity secret in localStorage; use reviewed encrypted custody or keep the secret out of persistent state.
• Define a non-destructive migration and user-visible cleanup path for existing plaintext preview records.
• Strictly validate identity import and make secret-bearing export an explicit warned action.
• Use the canonical BN254 MiMC identity commitment and chain-bound nullifier encoding.
• Block MsgRegisterIdentity from the preview client unless the complete approved custody and prover activation boundary is satisfied.
• Remove or qualify UI anonymity claims while submission is disabled.
• Add storage, reload, lock, import/export, malformed-data, commitment-parity and transaction-registry tests.
• Keep ZKPService.isSubmittable hard false until separate production gates pass.

Dependency

#300 may continue only in compatibility-only mode and may not claim completion of this custody/activation ticket.