feat(gl-sdk): add high-level auth API with Config, register, recover, connect

[angelix]

Summary

• Add a high-level auth API (register, recover, connect, register_or_recover) that replaces the multi-object ceremony (Signer → Scheduler → register/recover → build Node) with single function calls
• Add Config builder type for network selection and optional DeveloperCert
• Add Node.credentials() / Node.disconnect() and guard all RPC methods against use-after-disconnect
• Move signer execution to a shared tokio runtime (Handle::spawn) to avoid per-signer threads and block_on deadlocks

Motivation

The current low-level API requires apps to manually orchestrate 4+ objects to get a working node. This is error-prone and exposes internal details (credential serialization, signer lifecycle, runtime management) that most apps don't need to
think about. The high-level API reduces the happy path to:

let config = Config::new();
let node = register_or_recover(mnemonic, invite_code, &config)?;
let creds = node.credentials()?; // persist these
// ... later ...                                                                                                                                                                                                                                
let node = connect(mnemonic, saved_creds, &config)?;
                                                                                                                                                                                                                                                
The low-level API (Signer, Scheduler, Node::new) remains available and unchanged.

Test plan

- Python integration tests (libs/gl-sdk/tests/test_auth_api.py): Config builder, register, recover, connect, register_or_recover, disconnect idempotency, duplicate register error, credentials roundtrip, multiple simultaneous nodes,         
disconnect blocks RPC, low-level credentials still work
- Android instrumented tests (AuthApiTest.kt, NodeOperationsTest.kt): Config construction, bad mnemonic errors, register-or-recover flow, credentials roundtrip via connect, disconnect idempotency, invoice creation, low-level API still works
- Verify low-level Signer.start() still works (uses Handle::spawn internally now)