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.Core.Tests/AppSettingsTests.cs

@@ -0,0 +1,73 @@
+using System.Reflection;
+using BulkArchiveMailer.Core;
+using Xunit;
+
+namespace BulkArchiveMailer.Core.Tests
+{
+    /// <summary>
+    /// Covers the one AppSettingsData behavior that's easy to silently
+    /// break when adding a field: loading a config.json written before
+    /// FromAccountSmtpAddress existed must not throw, and a non-empty
+    /// value must round-trip once the operator picks a "from" account in
+    /// SettingsForm.
+    ///
+    /// Note the old-JSON case asserts null, not "": DataContractJsonSerializer
+    /// builds the target via FormatterServices.GetUninitializedObject,
+    /// which skips this property's "" initializer entirely for any member
+    /// absent from the source JSON, leaving the CLR default (null) - see
+    /// the long comment on AppSettingsData.FromAccountSmtpAddress. Every
+    /// call site (MailSender, SettingsForm) already treats null the same
+    /// as "" for this reason.
+    ///
+    /// Goes through AppSettings' private Serialize/Deserialize via
+    /// reflection rather than Load()/Save(), so this test never touches
+    /// the real %APPDATA%\BulkArchiveMailer\config.json on the machine
+    /// running it.
+    /// </summary>
+    public class AppSettingsTests
+    {
+        [Fact]
+        public void Deserialize_OldJsonWithoutFromAccountField_DoesNotThrowAndTreatsAsUnset()
+        {
+            const string oldJson = @"{
+                ""archiveFolder"": ""C:\\archives"",
+                ""emailDomain"": ""redlab.lo"",
+                ""testModeDefault"": true,
+                ""signatureHtmlPath"": """"
+            }";
+
+            var settings = InvokeDeserialize(oldJson);
+
+            Assert.True(string.IsNullOrEmpty(settings.FromAccountSmtpAddress));
+            Assert.Equal("redlab.lo", settings.EmailDomain);
+        }
+
+        [Fact]
+        public void SerializeThenDeserialize_RoundTripsFromAccountSmtpAddress()
+        {
+            var data = new AppSettingsData
+            {
+                ArchiveFolder = "C:\\archives",
+                EmailDomain = "redlab.lo",
+                FromAccountSmtpAddress = "svc-mailer@redlab.lo",
+            };
+
+            var json = InvokeSerialize(data);
+            var roundTripped = InvokeDeserialize(json);
+
+            Assert.Equal("svc-mailer@redlab.lo", roundTripped.FromAccountSmtpAddress);
+        }
+
+        private static AppSettingsData InvokeDeserialize(string json)
+        {
+            var method = typeof(AppSettings).GetMethod("Deserialize", BindingFlags.NonPublic | BindingFlags.Static);
+            return (AppSettingsData)method.Invoke(null, new object[] { json });
+        }
+
+        private static string InvokeSerialize(AppSettingsData data)
+        {
+            var method = typeof(AppSettings).GetMethod("Serialize", BindingFlags.NonPublic | BindingFlags.Static);
+            return (string)method.Invoke(null, new object[] { data });
+        }
+    }
+}