fix: use P-256 access keys for MPP payments

[decofe]

Summary

• resolve MPP payments with locally stored access-key identities before the generic provider fallback
• rehydrate CLI P-256 JWK handles through the accounts WebCrypto keystore
• add a regression test for the persisted passkey-backed key format

Root cause

resolvePaymentIdentity() never used storedAccessKeyIdentity(), and the reconstruction helper only accepted legacy privateKey records. CLI-created P-256 keys are persisted as handle.jwk plus publicKey, so payment creation fell back to a generic JSON-RPC identity and crashed with Cannot read properties of undefined (reading 'toLowerCase').

Validation

• pnpm check:format
• pnpm check:lint
• pnpm check:types
• pnpm test — 112 passed
• live Browserbase reproduction now reaches the expected InsufficientBalance revert instead of the toLowerCase exception

Fixes #49

Prompted by: @alexrisch