GDrive: add optional encryption checkbox for backup files

[altoch]

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• I opened an issue first for non-trivial changes and linked it below.
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Related issue
No issue opened. The change is self-contained and the problem is described below.

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Describe the problem
When the Password field is left empty in the Google Drive backup settings, encrypt() fails silently because OpenSSL receives an empty password file. As a result, encrypt() returns null, which is passed directly to $client->upload(). The file is created on Google Drive with 0 bytes, and the UI still reports "Backup successful" with no indication of failure.

Additionally, encryption should be opt-in. There are valid use cases where a user may not want to encrypt backups (e.g. private Shared Drives with restricted access), and forcing encryption without a clear UI option is inconsistent with the rest of the backup UI (Download and Restore sections already use an explicit encrypt/decrypt checkbox).

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Describe the proposed solution
Added an optional "Encrypt backup files" checkbox to the Google Drive backup configuration, consistent with the existing Download and Restore UI patterns.

• If the checkbox is unchecked: the configuration is uploaded in plaintext. No password required.
• If the checkbox is checked and a password is provided: the configuration is encrypted before upload.
• If the checkbox is checked but no password is provided: encryption fails, the backup is aborted, and an error is logged. No 0-byte file is uploaded.

The comparison logic when checking for duplicate backups also respects the encryption setting, decrypting the previous backup only when encryption is enabled.