feat(sdk): Add resumeAuthFlow to recover auth flows after page refresh

[tipogi]

Summary

• Add resume_auth_flow(url) to the core Pubky facade and JS/WASM bindings, allowing an app to reconnect to an in-progress auth flow using a previously saved authorizationUrl
• Parse the secret, capabilities, relay, and flow kind from the persisted URL and rebuild the PubkyAuthFlow against the same relay channel — if the signer already approved, the first poll returns a session immediately
• Add Rust unit tests (signin + signup resume, invalid URL rejection, seed-export rejection) and JS integration tests covering the full round-trip including .free() to simulate WASM memory loss