Fix hidden add/remove template buttons, add per-template login filter

The add/remove template buttons sat directly below the fixed-height
list with only 6px of margin to the tab's bottom edge - enough on this
dev machine but not on the operator's real one (different DPI/font
scale), so the buttons rendered off the visible area. Moved them above
the list instead (visibility no longer depends on how much space is
left at the bottom) and gave the whole "Шаблоны" tab substantially
more vertical room throughout.

Also added an optional per-template login regex filter
(MailTemplateDefinition.LoginPattern): once a file's suffix matches a
template, its login must also match that template's pattern (if any)
or the row surfaces as a clear Error in the preview instead of
silently going through - addresses "непонятно что будет работать".

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

outlook-bulk-sender/BulkArchiveMailer.Core/AppSettings.cs

@@ -24,6 +24,10 @@ namespace BulkArchiveMailer.Core
 
         [DataMember(Name = "bodyHtml")]
         public string BodyHtml { get; set; } = "";
+
+        /// <summary>See MailTemplateDefinition.LoginPattern. Absent from any config.json written before v1.2.0 - DataContractJsonSerializer leaves this null (not "") for those, same caveat as every other string member here; ToTemplateList/SetTemplates and every call site normalize via `?? ""`.</summary>
+        [DataMember(Name = "loginPattern")]
+        public string LoginPattern { get; set; } = "";
     }
 
     /// <summary>Shape of %APPDATA%\BulkArchiveMailer\config.json (and of the embedded config.default.json that seeds it).</summary>
@@ -95,6 +99,7 @@ namespace BulkArchiveMailer.Core
                     FileSuffix = t?.FileSuffix ?? "",
                     Subject = t?.Subject ?? "",
                     BodyHtml = t?.BodyHtml ?? "",
+                    LoginPattern = t?.LoginPattern ?? "",
                 });
             }
 
@@ -115,6 +120,7 @@ namespace BulkArchiveMailer.Core
                         FileSuffix = t?.FileSuffix ?? "",
                         Subject = t?.Subject ?? "",
                         BodyHtml = t?.BodyHtml ?? "",
+                        LoginPattern = t?.LoginPattern ?? "",
                     });
                 }
             }
@@ -380,6 +386,11 @@ namespace BulkArchiveMailer.Core
                         FileSuffix = "ssh_access",
                         Subject = legacy?.Templates?.SshAccess?.Subject ?? "",
                         BodyHtml = legacy?.Templates?.SshAccess?.BodyHtml ?? "",
+                        // The legacy shape never had a per-template login
+                        // filter - migrated entries keep the old
+                        // unrestricted "suffix alone decides the match"
+                        // behavior.
+                        LoginPattern = "",
                     },
                     new MailTemplateData
                     {
@@ -387,6 +398,7 @@ namespace BulkArchiveMailer.Core
                         FileSuffix = "config_update",
                         Subject = legacy?.Templates?.ConfigUpdate?.Subject ?? "",
                         BodyHtml = legacy?.Templates?.ConfigUpdate?.BodyHtml ?? "",
+                        LoginPattern = "",
                     },
                 },
             };