targets .NET 8.0 + .NET Standard 2.0, C# 12

addresses several security and correctness issues found in a code audit:

BinaryFormatter replaced with System.Text.Json — BinaryFormatter is deprecated (SYSLIB0011) and vulnerable to arbitrary code execution (CWE-502)
Persistence no longer hardcoded to Enterprise — credentials are no longer silently synced to domain controllers (fixes #69)
JIT-safe memory zeroing — credential buffers are zeroed via RtlZeroMemory P/Invoke before CredFree()
P/Invoke fixes — missing SetLastError, corrected buffer sizes
Also modernized: targets .NET 8.0 + .NET Standard 2.0, C# 12, nullable annotations, i18n (en/de/fr/es/it).

Initial release

Initial release with support to get credentials from user, store and retrieve it from Credential vault in Windows.