feat: Implement admin-only specialties management interface

- Added MailerAttachmentPolicy for validating email attachments against server rules.
- Introduced fis.specialties.js for managing hidden maintenance actions via an admin interface.
- Created fis.specialties.scss for styling the specialties management UI.
- Added German localization for specialties management in fis.specialties_txt_de.js.
- Developed CSS for specialties management in fis.specialties.css.
- Implemented server-side stored procedure fds__admin_backupAndClearInvoiceFile for backing up and clearing invoice files.
- Created fds__invoice_file_backups table to store backup records of invoice files.

Fuchs_Database/FuchsDatabase.sqlproj

@@ -127,6 +127,7 @@
     <Build Include="dbo\Tables\fds__mfr_invoicefiles.sql" />
     <Build Include="dbo\Tables\fds__log.sql" />
     <Build Include="dbo\Tables\fds__invoices.sql" />
+    <Build Include="dbo\Tables\fds__invoice_file_backups.sql" />
     <Build Include="dbo\Tables\fds__invoice_servicerequests.sql" />
     <Build Include="dbo\Tables\fds__invoice_items.sql" />
     <Build Include="dbo\Tables\fds__invoice_details.sql" />
@@ -357,6 +358,7 @@
     <Build Include="dbo\Stored Procedures\fds__admin_logdebug.sql" />
     <Build Include="dbo\Stored Procedures\fds__admin_logdebug_ext.sql" />
     <Build Include="dbo\Stored Procedures\fds__admin_getReportCatalog.sql" />
+    <Build Include="dbo\Stored Procedures\fds__admin_backupAndClearInvoiceFile.sql" />
     <Build Include="dbo\Stored Procedures\_BackupAndClearInvoiceFile.sql" />
     <Build Include="dbo\User Defined Types\json_data_maxU.sql" />
     <Build Include="dbo\User Defined Types\hash_256.sql" />