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/OutlookAccounts.cs

@@ -0,0 +1,77 @@
+using System;
+using System.Collections.Generic;
+using System.Runtime.InteropServices;
+using Outlook = Microsoft.Office.Interop.Outlook;
+
+namespace BulkArchiveMailer.AddIn
+{
+    /// <summary>Read-only snapshot of one Outlook.Account, taken once while the COM object was still alive - safe to hold onto and display after the Account RCW itself has been released.</summary>
+    public sealed class OutlookAccountInfo
+    {
+        public string DisplayName { get; set; } = "";
+        public string SmtpAddress { get; set; } = "";
+    }
+
+    /// <summary>
+    /// Enumerates Outlook.Application.Session.Accounts for UI purposes
+    /// (the "Отправлять от:" picker in SettingsForm) without leaking any
+    /// COM references - mirrors the release-what-you-touch convention used
+    /// in MailSender/SignatureReader. Never throws: on any failure it
+    /// returns an empty list so a picker can simply fall back to "default
+    /// account only" instead of crashing the Settings window.
+    /// </summary>
+    public static class OutlookAccounts
+    {
+        public static List<OutlookAccountInfo> ListAccounts(Outlook.Application application)
+        {
+            var result = new List<OutlookAccountInfo>();
+            if (application == null)
+            {
+                return result;
+            }
+
+            Outlook.Accounts accounts = null;
+            try
+            {
+                accounts = application.Session.Accounts;
+                foreach (Outlook.Account account in accounts)
+                {
+                    try
+                    {
+                        result.Add(new OutlookAccountInfo
+                        {
+                            DisplayName = account.DisplayName ?? "",
+                            SmtpAddress = account.SmtpAddress ?? "",
+                        });
+                    }
+                    catch
+                    {
+                        // Skip an individual account we couldn't read
+                        // rather than losing the rest of the list.
+                    }
+                    finally
+                    {
+                        Marshal.ReleaseComObject(account);
+                    }
+                }
+            }
+            catch
+            {
+                return new List<OutlookAccountInfo>();
+            }
+            finally
+            {
+                if (accounts != null)
+                {
+                    Marshal.ReleaseComObject(accounts);
+                }
+
+                // application.Session (NameSpace) is a long-lived object
+                // owned by Outlook itself - never released here, same as
+                // in MailSender.
+            }
+
+            return result;
+        }
+    }
+}