Passkey smart-account wallet mode

[truthixify]

Tier: L (5-7 days) | Type: feature

Context. The SDK now exposes WebAuthnPasskeyStealthSigner in sdk/src/chains/stellar/signer.ts, but the demo does not consume it yet: src/pages/Settings.tsx only surfaces the theme preference, and none of the five existing Stellar adapters (Freighter, Albedo, xBull, LOBSTR, WalletConnect) is a passkey peer. Wave 8 introduces passkey as a first-class wallet mode: the app can operate entirely from a passkey-controlled Stellar smart account, sponsor its own fees, and never touch an extension after setup.

Scope.

• New PasskeyAdapter under src/wallets/stellar/ implementing the shared adapter interface.
• First-run flow: create-or-import a smart account, register the passkey (PRF extension), fund via friendbot on testnet or a launch-day sponsor on mainnet.
• Session-key pattern for repeated ops within one browser session (with a documented ceiling).
• Graceful error state when the browser has no PRF-capable authenticator: render a clear next-step card explaining the requirement and pointing to a supported browser or hardware key.
• Recording of the full flow on Chrome desktop, iOS Safari, and Android Chrome.

Acceptance.

• Wallet picker lists Passkey as a peer of Freighter / Albedo / xBull / LOBSTR
• Fresh account can complete stealth send and stealth withdraw without any extension prompt
• PRF-absent state renders a clear next-step card, never a silent failure
• Adapter compiles without @ts-nocheck and has a unit test around the PRF assertion parser

Files. src/wallets/stellar/PasskeyAdapter.ts (new), src/wallets/stellar/index.ts, src/components/StellarWalletPicker.tsx, src/lib/stellar/passkey.ts.