Active Directory account pickers

• Services/ActiveDirectoryService.cs enumerates every domain in the current forest from the RootDSE defaultNamingContext and the Partitions container's domain crossRefs, so there is no dependency on System.DirectoryServices.ActiveDirectory. A domain's users are searched with a single LDAP-escaped DirectorySearcher matching sAMAccountName, displayName, mail and userPrincipalName.
• It is built on System.DirectoryServices, which net8.0-windows resolves from the Windows Desktop shared framework the installer already bundles. On a machine that is not domain-joined it reports the reason instead of throwing.
• Account then Directory carries "Browse Active Directory…". The picked account fills ADDomain and ADUsername and ticks linked to Active Directory. Distribution-list recipients gained "Add from AD…", which multi-selects accounts and imports their e-mail addresses into the list.
• Validated end to end against a live domain controller (progressiverobot.local): domain enumeration, the all-users query and the name and e-mail filters all returned the expected results.

Other changes

• A generic TargetInvocationException from the server is now unwrapped and the real cause explained: connect with the server-administrator account, or the server cannot reach its database. That is what produced the misleading "26 settings could not be read" message on the Protocols page over a non-admin connection.
• The vendored MariaDB Connector/C client (libmysql.dll and its auth plugins) is staged into the build output by the server post-build step, so MySQL and MariaDB back ends work out of the box.
• The About page carries maintainer and company details.

The Control Panel requires the .NET 8 Desktop Runtime, which the installer bundles and installs silently when missing.

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