feat(stellar): add passkey smart-account wallet mode

[collinsezedike]

Summary

Closes #150.

• Adds browser-side WebAuthn PRF plumbing (src/lib/stellar/passkey.ts): credential creation, re-authentication, PRF extension result parsing, and a documented session ceiling (30 min or 20 signatures, whichever comes first) on how long the derived key stays resident in memory.
• Adds PasskeyAdapter implementing the shared StellarWallet interface, registered alongside Freighter/Albedo/xBull/LOBSTR/WalletConnect in src/wallets/stellar/index.ts. First-run creates a passkey, deterministically derives a Stellar Ed25519 keypair from its PRF secret, and funds it via friendbot on testnet — no extension prompt at any point.
• Adds a PasskeyUnsupportedCard next-step card, rendered by StellarWalletPicker in place of the generic error string when a device can't complete the PRF ceremony, pointing at passkeys.dev's device-support page.
• Unit tests for the PRF extension result parser, base64url helpers, and the session ceiling.

Important scope correction (read this before reviewing)

The issue describes a Soroban smart account with on-chain fee sponsorship and a contract-delegated session key, authorized via an SDK export called WebAuthnPasskeyStealthSigner. I checked every version of @wraith-protocol/sdk published to npm (1.0.0 through the current latest, 1.4.5) — that export does not exist anywhere. A real smart-contract account would also need a deployed Soroban wallet contract (Rust/WASM) that doesn't exist anywhere in this repo.

Since neither the SDK export nor a deployed contract exist, this PR ships an honestly-scoped-down but fully working alternative: a classic Ed25519 Stellar account whose signing key is deterministically derived from the passkey's PRF secret via Keypair.fromRawEd25519Seed (@stellar/stellar-sdk, already a dependency — no new packages added). It satisfies "no extension prompt, funds itself on testnet, PRF-gated, session ceiling" end to end with real, working code. What it does not do is on-chain fee sponsorship or contract-delegated session keys — those need the missing smart-wallet contract and are natural follow-up work once one exists. See the scope note at the top of PasskeyAdapter.ts for the full reasoning.

Pre-existing gap found during this work

StellarWalletPicker and StellarWalletButton (the multi-wallet adapter system this issue extends) are not currently mounted anywhere in the app — Header.tsx renders WalletConnect.tsx, which uses a separate, Freighter-only StellarWalletContext that predates the adapter registry. This means Albedo, xBull, LOBSTR, and WalletConnect were already unreachable in the running app before this PR, and Passkey inherits that same gap. I scoped this PR to exactly the files the issue lists rather than also rewiring Header.tsx off the legacy context, since that touches shared wallet state consumed by Send/Receive/Vault/batch-withdraw/notifications. Flagging for a maintainer decision on whether that's a separate issue.

Acceptance criteria status

• Wallet picker lists Passkey as a peer of Freighter / Albedo / xBull / LOBSTR (structurally — see reachability gap above)
• First-run flow signs with no browser-extension prompt
• PRF-absent state renders a next-step card, not a silent failure
• Adapter compiles without @ts-nocheck; PRF assertion parser has unit tests
• "Smart account" / fee sponsorship / contract session-key delegation — descoped, see correction above; needs a deployed Soroban wallet contract
• Cross-browser recordings (Chrome desktop, iOS Safari, Android Chrome) — not produced in this environment; needs real devices/browsers

Test plan

• pnpm test:unit for src/lib/stellar/passkey.test.ts
• Manual: mount StellarWalletButton/StellarWalletPicker (or a temporary harness) to click through first-run passkey creation, friendbot funding, and a stealth send/withdraw on testnet
• Manual: verify the PRF-unsupported card on a browser/device without PRF support
• Manual: verify a returning session re-derives the same address and that signing after the 30 min / 20 signature ceiling re-prompts the passkey

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[truthixify]

Merged. Dedicated tests around the PRF assertion parser and a real unsupported-authenticator card rather than a silent failure. Careful piece of work @collinsezedike.