Fix crash opening bulk sender window (rows null during InitializeComponent)

Real stack trace from the operator's machine pinned it down: WinForms
fires DataGridView.CellValueChanged as a side effect of
DataGridViewColumn.HeaderText assignments inside InitializeComponent(),
i.e. before PreviewForm's constructor has had a chance to run RunScan()
and populate the `rows` field. UpdateSummaryLabel() then hit
rows.Count(...) on a null source. Nothing to do with Outlook/COM at
all - a pure WinForms initialization-order gotcha.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

outlook-bulk-sender/BulkArchiveMailer.AddIn/UI/PreviewForm.cs

@@ -128,6 +128,17 @@ namespace BulkArchiveMailer.AddIn.UI
 
         private void UpdateSummaryLabel()
         {
+            if (rows == null)
+            {
+                // Can be invoked before RunScan() ever ran - most notably,
+                // WinForms fires DataGridView.CellValueChanged as a side
+                // effect of DataGridViewColumn.HeaderText assignments in
+                // InitializeComponent(), i.e. before the constructor (and
+                // therefore RunScan()) has had a chance to populate `rows`.
+                // Nothing to summarize yet in that case.
+                return;
+            }
+
             var ready = rows.Count(r => r.Job.Status == SendStatus.Ready);
             var already = rows.Count(r => r.Job.Status == SendStatus.AlreadySent);
             var errors = rows.Count(r => r.Job.Status == SendStatus.Error);