Add biometrics-gated export for stealth key backup

[collinsadi]

Encrypted export still accessible to anyone with unlocked device session.

Optional WebAuthn or platform authenticator gate before decrypting or exporting stealth keys.

Acceptance criteria:

• Biometric gate works on supported platforms.
• Graceful fallback with passphrase.
• Documented in security settings.

[akinboyewaSamson]

@akinboyewaSamson has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi! I’m a full-stack developer with a strong blockchain background. I’d love to handle this issue and can deliver a clean, reliable solution. Kindly assign it to me.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @akinboyewaSamson to this issue.