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/deploy/install-addin.reg

@@ -0,0 +1,12 @@
+Windows Registry Editor Version 5.00
+
+; Sideload регистрация BulkArchiveMailer для текущего пользователя.
+; Перед применением скопируйте всю папку bin\Release\ в C:\Tools\BulkArchiveMailer\
+; (или другой стабильный путь - тогда поправьте "Manifest" ниже под свой путь).
+; Подробности и типичные проблемы - outlook-bulk-sender/docs/BUILD.md.
+
+[HKEY_CURRENT_USER\Software\Microsoft\Office\Outlook\Addins\BulkArchiveMailer.AddIn]
+"Description"="Массовая рассылка архивов ssh-access-tui"
+"FriendlyName"="BulkArchiveMailer"
+"Manifest"="file:///C:\\Tools\\BulkArchiveMailer\\BulkArchiveMailer.AddIn.vsto|vstolocal"
+"LoadBehavior"=dword:00000003