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/UI/SettingsForm.cs

@@ -42,6 +42,16 @@ namespace BulkArchiveMailer.AddIn.UI
             PopulateFrom(settings);
         }
 
+        /// <summary>ComboBox item: displays a friendly label but carries the SmtpAddress that actually gets saved to config.json. The first item (SmtpAddress == "") represents "let Outlook pick its default account".</summary>
+        private sealed class FromAccountItem
+        {
+            public string Label { get; set; } = "";
+            public string SmtpAddress { get; set; } = "";
+            public override string ToString() => Label;
+        }
+
+        private const string DefaultAccountLabel = "(по умолчанию Outlook)";
+
         private void PopulateFrom(AppSettingsData settings)
         {
             txtArchiveFolder.Text = settings.ArchiveFolder ?? "";
@@ -49,6 +59,8 @@ namespace BulkArchiveMailer.AddIn.UI
             txtSignaturePath.Text = settings.SignatureHtmlPath ?? "";
             chkTestModeDefault.Checked = settings.TestModeDefault;
 
+            PopulateFromAccountCombo(settings.FromAccountSmtpAddress ?? "");
+
             var templates = settings.ToTemplateSet();
             txtSshSubject.Text = templates.SshAccess.Subject;
             txtSshBody.Text = templates.SshAccess.BodyHtml;
@@ -56,6 +68,49 @@ namespace BulkArchiveMailer.AddIn.UI
             txtCfgBody.Text = templates.ConfigUpdate.BodyHtml;
         }
 
+        /// <summary>
+        /// Fills cboFromAccount with "(по умолчанию Outlook)" plus one
+        /// entry per real Outlook.Account in the current session, then
+        /// selects the item matching savedSmtpAddress. If that address no
+        /// longer belongs to any account in this session (removed/renamed
+        /// since it was configured, or Outlook.Application is unavailable
+        /// e.g. under a design-time host), it just falls back to the
+        /// default item instead of throwing - never blocks opening the
+        /// Settings window over this.
+        /// </summary>
+        private void PopulateFromAccountCombo(string savedSmtpAddress)
+        {
+            cboFromAccount.Items.Clear();
+            cboFromAccount.Items.Add(new FromAccountItem { Label = DefaultAccountLabel, SmtpAddress = "" });
+
+            if (outlookApplication != null)
+            {
+                foreach (var account in OutlookAccounts.ListAccounts(outlookApplication))
+                {
+                    var label = string.IsNullOrEmpty(account.SmtpAddress)
+                        ? account.DisplayName
+                        : account.DisplayName + " <" + account.SmtpAddress + ">";
+                    cboFromAccount.Items.Add(new FromAccountItem { Label = label, SmtpAddress = account.SmtpAddress });
+                }
+            }
+
+            var selectedIndex = 0;
+            if (!string.IsNullOrEmpty(savedSmtpAddress))
+            {
+                for (var i = 0; i < cboFromAccount.Items.Count; i++)
+                {
+                    var item = (FromAccountItem)cboFromAccount.Items[i];
+                    if (string.Equals(item.SmtpAddress, savedSmtpAddress, StringComparison.OrdinalIgnoreCase))
+                    {
+                        selectedIndex = i;
+                        break;
+                    }
+                }
+            }
+
+            cboFromAccount.SelectedIndex = selectedIndex;
+        }
+
         private void btnBrowseFolder_Click(object sender, EventArgs e)
         {
             using (var dialog = new FolderBrowserDialog())
@@ -157,12 +212,15 @@ namespace BulkArchiveMailer.AddIn.UI
                 }
             }
 
+            var selectedAccount = cboFromAccount.SelectedItem as FromAccountItem;
+
             var data = new AppSettingsData
             {
                 ArchiveFolder = txtArchiveFolder.Text.Trim(),
                 EmailDomain = txtDomain.Text.Trim(),
                 SignatureHtmlPath = txtSignaturePath.Text.Trim(),
                 TestModeDefault = chkTestModeDefault.Checked,
+                FromAccountSmtpAddress = selectedAccount?.SmtpAddress ?? "",
             };
             data.SetTemplate(ArchiveKind.SshAccess, new MailTemplate { Subject = txtSshSubject.Text, BodyHtml = txtSshBody.Text });
             data.SetTemplate(ArchiveKind.ConfigUpdate, new MailTemplate { Subject = txtCfgSubject.Text, BodyHtml = txtCfgBody.Text });