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.Designer.cs

@@ -1,24 +1,68 @@
 namespace BulkArchiveMailer.AddIn
 {
     /// <summary>
-    /// Designer-generated half of ThisAddIn. Normally Visual Studio
-    /// regenerates this file's contents automatically the first time the
-    /// project is opened/built (it derives from
-    /// Microsoft.Office.Tools.Outlook.OutlookAddIn and wires Startup/
-    /// Shutdown) - it is hand-written here only because this environment
-    /// has no VSTO tooling to run the code generator. Do not hand-edit
-    /// once opened in VS if the IDE regenerates it differently.
+    /// Designer-generated half of ThisAddIn. Normally Visual Studio's VSTO
+    /// project system regenerates this file automatically from
+    /// ThisAddIn.Designer.xml the first time the project is opened/edited
+    /// in the IDE (host-item "blueprint" mechanism) - it is hand-written
+    /// here only because this environment had no VSTO tooling to run that
+    /// generator, and later corrected to match the real base class after
+    /// the first build against the actual VSTO assemblies (see git log).
+    ///
+    /// The real base class is NOT the <c>Microsoft.Office.Tools.Outlook.OutlookAddIn</c>
+    /// interface (that only declares the FormRegion-related members) but
+    /// <c>Microsoft.Office.Tools.Outlook.OutlookAddInBase</c> - a concrete
+    /// abstract class living in Microsoft.Office.Tools.Outlook.v4.0.Utilities.dll
+    /// (referenced with SpecificVersion=False/Private=True in the .csproj,
+    /// same as the built-in "Outlook VSTO Add-in" project template), which
+    /// itself derives from Microsoft.Office.Tools.AddInBase and already
+    /// implements OutlookAddIn/AddIn/IBindableComponent/IComponent/
+    /// IDisposable, plus exposes the protected virtual
+    /// CreateRibbonExtensibilityObject() overridden in ThisAddIn.cs.
+    ///
+    /// Do not hand-edit once opened in VS if the IDE regenerates it
+    /// differently.
     /// </summary>
-    partial class ThisAddIn : Microsoft.Office.Tools.Outlook.OutlookAddIn
+    [Microsoft.VisualStudio.Tools.Applications.Runtime.StartupObjectAttribute(0)]
+    [global::System.Security.Permissions.PermissionSetAttribute(global::System.Security.Permissions.SecurityAction.Demand, Name = "FullTrust")]
+    public sealed partial class ThisAddIn : Microsoft.Office.Tools.Outlook.OutlookAddInBase
     {
+        /// <summary>Populated from the host item in <see cref="Initialize"/>; exposed to the rest of the add-in via the OutlookApplication alias in ThisAddIn.cs.</summary>
+        internal Microsoft.Office.Interop.Outlook.Application Application;
+
+        /// <summary>
+        /// Required constructor for VSTO add-in support - do not modify
+        /// the contents of this method with the code editor. The VSTO
+        /// runtime instantiates ThisAddIn through this constructor via
+        /// reflection (see the StartupObjectAttribute above).
+        /// </summary>
+        public ThisAddIn(Microsoft.Office.Tools.Outlook.Factory factory, System.IServiceProvider serviceProvider)
+            : base(factory, serviceProvider, "AddIn", "ThisAddIn")
+        {
+            Globals.Factory = factory;
+        }
+
         /// <summary>
         /// Required method for Designer support - do not modify the
         /// contents of this method with the code editor.
         /// </summary>
-        private void InternalStartup()
+        protected override void Initialize()
         {
-            this.Startup += new System.EventHandler(ThisAddIn_Startup);
-            this.Shutdown += new System.EventHandler(ThisAddIn_Shutdown);
+            base.Initialize();
+            this.Application = this.GetHostItem<Microsoft.Office.Interop.Outlook.Application>(
+                typeof(Microsoft.Office.Interop.Outlook.Application), "Application");
+            Globals.ThisAddIn = this;
+            global::System.Windows.Forms.Application.EnableVisualStyles();
+        }
+
+        /// <summary>
+        /// Required method for Designer support - do not modify the
+        /// contents of this method with the code editor.
+        /// </summary>
+        protected override void FinishInitialization()
+        {
+            this.InternalStartup();
+            this.OnStartup();
         }
     }
 }