Implement passkeys to unlock the keystore (alongside or instead of PIN)

[dmnyc]

Currently the encrypted keystore is unlocked with a PIN. Passkeys (WebAuthn) should be supported as an additional — or replacement — unlock factor.

Why

• PINs are guessable, shoulder-surfable, and reused across sites; passkeys are phishing-resistant and bound to the device.
• A browser extension sidebar can use WebAuthn with publickey-credentials in the manifest permissions (Chromium supports this for extensions with MV3).
• Aligns with the 2026-08-14 security audit's general posture of hardening the unlock path (cf. Security audit 2026-08-14 remediation (blocks 1.9.0 release) #199 remediation batch).

Options to decide

1. Passkey alongside PIN — user picks either factor at unlock; PIN remains as fallback (safer rollout, but keeps the weaker factor alive).
2. Passkey replaces PIN — WebAuthn PRF extension derives/decrypts the keystore key directly (no password in the loop at all); requires a recovery path (e.g. ncryptsec backup) for lost devices.

Implementation notes

• Research: WebAuthn PRF (prf extension) support in Chromium extension sidepanels — can we use the PRF output to derive the AES key that currently wraps the keystore?
• If PRF is unavailable/awkward: store a random unlock secret in the keystore wrapper, gate its release behind navigator.credentials.get() with user verification
• Manifest: add publickey-credentials permission; verify behavior in Firefox (may need a PIN fallback path there)
• Enrollment flow: settings pane to register a passkey (with device/user verification required)
• Unlock flow: passkey prompt in the sidebar before decrypting the keystore
• Recovery: document that ncryptsec backup is the recovery path if the passkey/device is lost
• Decide option 1 vs 2 above (product decision, similar to Product decision: should the trusted tier keep silent nip04/nip44 decrypt (DM reading)? #197/Product decision: nostrarchives follow-list disclosure (opt-in setting, document, or drop) #194)

Audit doc: docs/security-audit-2026-08-14.md