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/MailSender.cs

@@ -28,6 +28,7 @@ namespace BulkArchiveMailer.AddIn
     {
         private readonly Outlook.Application application;
         private readonly string signatureHtml;
+        private readonly string fromAccountSmtpAddress;
 
         /// <param name="application">Live Outlook.Application (from ThisAddIn.Application) used to create mail items.</param>
         /// <param name="signatureHtml">
@@ -37,10 +38,18 @@ namespace BulkArchiveMailer.AddIn
         /// appended to HTMLBody by hand). May be null/empty if no
         /// signature is configured; the mail is then sent without one.
         /// </param>
-        public MailSender(Outlook.Application application, string signatureHtml)
+        /// <param name="fromAccountSmtpAddress">
+        /// SmtpAddress of the Outlook account (from Session.Accounts) to
+        /// send from, e.g. a shared mailbox distinct from the operator's
+        /// own primary account (AppSettingsData.FromAccountSmtpAddress).
+        /// Null/empty means "use whatever account Outlook picks by
+        /// default" - the previous, unconfigurable behavior.
+        /// </param>
+        public MailSender(Outlook.Application application, string signatureHtml, string fromAccountSmtpAddress = null)
         {
             this.application = application ?? throw new ArgumentNullException(nameof(application));
             this.signatureHtml = signatureHtml ?? string.Empty;
+            this.fromAccountSmtpAddress = fromAccountSmtpAddress ?? string.Empty;
         }
 
         /// <param name="job">Row to send. Caller is responsible for only passing selectable (Ready/AlreadySent), checked rows - this method does not re-validate Status.</param>
@@ -59,6 +68,26 @@ namespace BulkArchiveMailer.AddIn
                 mailItem.To = job.Email;
                 mailItem.Subject = job.Subject;
 
+                if (!string.IsNullOrEmpty(fromAccountSmtpAddress))
+                {
+                    var account = TryFindAccountBySmtpAddress(fromAccountSmtpAddress);
+                    if (account != null)
+                    {
+                        try
+                        {
+                            mailItem.SendUsingAccount = account;
+                        }
+                        finally
+                        {
+                            Marshal.ReleaseComObject(account);
+                        }
+                    }
+                    // Account no longer present in this Outlook session
+                    // (removed/renamed since it was configured) - fall
+                    // through and let Outlook use its own default account,
+                    // same as when the setting is empty.
+                }
+
                 var body = job.BodyHtml ?? string.Empty;
                 if (!string.IsNullOrEmpty(signatureHtml))
                 {
@@ -95,5 +124,63 @@ namespace BulkArchiveMailer.AddIn
                 }
             }
         }
+
+        /// <summary>
+        /// Looks up a live Outlook.Account in the current session by
+        /// SmtpAddress (case-insensitive). Returns null - never throws -
+        /// if the account isn't found or Session/Accounts can't be read
+        /// for any reason, so a stale/renamed configured account degrades
+        /// to "use Outlook's default" instead of failing the send.
+        ///
+        /// Ownership: the returned Account (if any) is a fresh RCW the
+        /// caller is responsible for releasing via
+        /// Marshal.ReleaseComObject once done with it, matching the
+        /// release-what-you-touch convention used throughout this class
+        /// and in SignatureReader.
+        /// </summary>
+        private Outlook.Account TryFindAccountBySmtpAddress(string smtpAddress)
+        {
+            Outlook.Accounts accounts = null;
+            try
+            {
+                accounts = application.Session.Accounts;
+                foreach (Outlook.Account candidate in accounts)
+                {
+                    bool isMatch;
+                    try
+                    {
+                        isMatch = string.Equals(candidate.SmtpAddress, smtpAddress, StringComparison.OrdinalIgnoreCase);
+                    }
+                    catch
+                    {
+                        isMatch = false;
+                    }
+
+                    if (isMatch)
+                    {
+                        return candidate; // ownership passed to the caller
+                    }
+
+                    Marshal.ReleaseComObject(candidate);
+                }
+
+                return null;
+            }
+            catch
+            {
+                return null;
+            }
+            finally
+            {
+                if (accounts != null)
+                {
+                    Marshal.ReleaseComObject(accounts);
+                }
+
+                // application.Session (the NameSpace) is a long-lived
+                // object owned by Outlook itself - like `application`, it
+                // is never released here.
+            }
+        }
     }
 }