test(labels): add comprehensive test suite for wallet labels

- Add tests for label persistence and deletion across reloads
- Add import/export tests for JSONL format
- Add encryption/decryption roundtrip tests
- Add validation tests for timestamps, rates, and field types
- Cover all label types: tx, addr, pubkey, input, output, xpub
- Add error handling tests for invalid JSON and missing fields

Author: Leeyah-123