🔒 Fix Unauthenticated Encryption Mode

[LDzik]

🎯 What: Replaced the unauthenticated AES-CBC encryption mode with the authenticated AES-GCM mode in UserManager.cs.

⚠️ Risk: AES-CBC without a Message Authentication Code (MAC) is vulnerable to padding oracle attacks and ciphertext tampering, allowing attackers to potentially decrypt or modify sensitive data.

🛡️ Solution: Implemented AES-GCM using System.Security.Cryptography.AesGcm. The encrypted data now includes a 16-byte authentication tag concatenated to the ciphertext, ensuring both confidentiality and integrity. Updated all hardcoded credentials and call sites (in CredentialProvider.cs and Tile.cs) to use the new encryption format and nonce terminology. Verified the fix with a security test script confirming that tampering with the tag results in a decryption failure.

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PR created automatically by Jules for task 4715416594284475450 started by @LDzik

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