Fix timezone handling for form start/end datetime fields

[wotis]

• Fix 2-hour offset bug where forms disabled too early (UTC vs local time)
• Properly use WordPress timezone settings in FormSettings::convertFormatToTime()
• Add timezone conversion for display logic in cforms-options.php
• Fix datetime storage with correct timezone handling in lib_options_sub.php
• Resolve DateTime namespace conflicts by adding backslash prefix
• Tested with MESZ/CET and manual UTC offset configurations

Fixes issue where forms with end dates would deactivate at 22:00 instead of 00:00 when WordPress timezone was set to Europe/Berlin (MESZ).

[bgermann]

Thanks for all the changes. I will need some time to review them. Please adjust the PR's title to represent what is in it.