Fix first real VSTO build, add sender account selection, polish deploy

First live build against real Visual Studio/VSTO tooling surfaced two
issues in the hand-written AddIn scaffold: ThisAddIn derived from the
wrong base type (OutlookAddIn is an interface, not the concrete
OutlookAddInBase class), and VSTO Application projects cannot build
without a signed ClickOnce manifest, so SignManifests is now on with a
throwaway self-signed test certificate. Both Core/Core.Tests/AddIn now
build clean and Core.Tests passes.

Also: mail always sent from Outlook's default account with no way to
pick a different one (e.g. a shared mailbox) - added a "From account"
setting (Session.Accounts dropdown in SettingsForm, SendUsingAccount in
MailSender). And added deploy/ with both a local ClickOnce publish
script (real install/uninstall via Windows Apps, no manual registry
editing) and a manual registry sideload fallback for scripted rollouts.

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

outlook-bulk-sender/BulkArchiveMailer.AddIn/ThisAddIn.cs

@@ -28,5 +28,20 @@ namespace BulkArchiveMailer.AddIn
         {
             return new RibbonController();
         }
+
+        #region VSTO generated code
+
+        /// <summary>
+        /// Required method for Designer support - do not modify the
+        /// contents of this method with the code editor. Called from
+        /// ThisAddIn.Designer.cs's FinishInitialization() override.
+        /// </summary>
+        private void InternalStartup()
+        {
+            this.Startup += new EventHandler(ThisAddIn_Startup);
+            this.Shutdown += new EventHandler(ThisAddIn_Shutdown);
+        }
+
+        #endregion
     }
 }