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/Globals.cs

@@ -5,10 +5,50 @@ namespace BulkArchiveMailer.AddIn
     /// shape Visual Studio normally generates for VSTO projects (hand
     /// written here for the same reason as ThisAddIn.Designer.cs - see
     /// its header comment).
+    ///
+    /// <c>Microsoft.Office.Tools.Outlook.OutlookAddIn</c> has no static
+    /// "GetInstance" helper (it is just the FormRegion-related interface
+    /// implemented by the real base class, OutlookAddInBase) - the
+    /// running instance is instead captured directly by
+    /// ThisAddIn.Initialize() into <see cref="ThisAddIn"/> below, exactly
+    /// like the real VSTO-generated Globals class does via its
+    /// <c>Globals.ThisAddIn = this;</c> assignment.
     /// </summary>
     internal static class Globals
     {
-        internal static ThisAddIn ThisAddIn =>
-            (ThisAddIn)Microsoft.Office.Tools.Outlook.OutlookAddIn.GetInstance(typeof(ThisAddIn));
+        private static ThisAddIn _thisAddIn;
+        private static Microsoft.Office.Tools.Outlook.Factory _factory;
+
+        internal static ThisAddIn ThisAddIn
+        {
+            get { return _thisAddIn; }
+            set
+            {
+                if (_thisAddIn == null)
+                {
+                    _thisAddIn = value;
+                }
+                else
+                {
+                    throw new System.NotSupportedException();
+                }
+            }
+        }
+
+        internal static Microsoft.Office.Tools.Outlook.Factory Factory
+        {
+            get { return _factory; }
+            set
+            {
+                if (_factory == null)
+                {
+                    _factory = value;
+                }
+                else
+                {
+                    throw new System.NotSupportedException();
+                }
+            }
+        }
     }
 }