Update pem.go

[infinitydaemon]

• Replaced panic calls with structured error handling in helper functions.
• Used %w in fmt.Errorf for better error wrapping.
• Better readability
• Centralized redundant logic (e.g., PEM encoding/decoding and file writing) into toPEMBytes, decodePEMBlock, and writePEMToFile.
• Ensured os.O_TRUNC is used to overwrite files securely in writePEMToFile.
• Unified similar logic between public and private key handling.

[david415]

lol dude you can't change the copyright notice

// SPDX-FileCopyrightText: Copyright (c) 2023-2024 David Stainton
// SPDX-License-Identifier: AGPL-3.0-only
+// infinitydaemon @CWDSYSTEMS

wrong.