fix(webapp): use indigo for the column reorder drop indicator

The drop line was blue-500 while every selection control around it (the row
checkboxes, the dialog's radios) is indigo. Note bg-primary is apple/green in
this theme, so this uses indigo-500 directly — the token those controls use.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: samejr

apps/webapp/app/components/runs/v3/RunsDisplayOptions.tsx

@@ -337,7 +337,7 @@ function ColumnRow({
         onDrop();
       }}
     >
-      {isOver && <div className="absolute inset-x-0 top-0 h-0.5 bg-blue-500" />}
+      {isOver && <div className="absolute inset-x-0 top-0 h-0.5 bg-indigo-500" />}
       {/* Native label so the whole name area toggles the column, matching CheckboxWithLabel. */}
       <label
         className={cn(