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/BulkArchiveMailer.Core.Tests.csproj

@@ -12,7 +12,7 @@
   -->
 
   <PropertyGroup>
-    <TargetFramework>net48</TargetFramework>
+    <TargetFramework>net481</TargetFramework>
     <IsPackable>false</IsPackable>
     <LangVersion>7.3</LangVersion>
   </PropertyGroup>