Powermail 7.5.2 Security Backport

Unofficial security backport for Powermail 7.5.x.

This release mitigates CVE-2026-77136 / TYPO3-EXT-SA-2026-022,
a Fluid server-side template injection in mail-header processing.

Changes:

• Prevent visitor-controlled sender and receiver header values from being evaluated as Fluid.
• Keep configured Fluid usage for mail subjects and configured sender/receiver values.
• Update extension version to 7.5.2.

Compatibility:

• No database migration or form changes are required.
• Existing sender_name fields and markers remain supported.
• This is an unofficial legacy backport and not a replacement for upgrading to a supported Powermail version.