fix: add checkbox spacer to column header for alignment

Column header was missing the w-9 spacer that ticket rows have
for the batch checkbox. Added spacer so columns align properly.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

web/src/app/page.tsx

@@ -884,10 +884,10 @@ function TicketWorkbenchContent() {
               <>
                 {/* Column header */}
                 <div className={cn(
-                  "sticky top-0 z-10 flex border-b border-border bg-muted/70",
-                  density === "compact" ? "min-h-8" : "min-h-9"
-                )}
-                >
+                  "sticky top-0 z-10 flex border-b border-border bg-muted/50",
+                  density === "compact" ? "min-h-7" : "min-h-8"
+                )}>
+                  <div className="w-9 shrink-0" />
                   {availableColumns.filter((c) => c.visible).map((col) => (
                     <div
                       key={col.key}