fix(ui): remove kanban column border missed by earlier sweep

The mockup's kanban columns have no border, just a light grey
background -- this instance used multi-line sx formatting
(border/borderColor on separate lines) so it slipped past the earlier
single-line-substring grep sweep. Caught by a follow-up multi-line
search across the whole frontend for the same pattern; nothing else
turned up except a legitimate circular crop-tool boundary in
CropImageDialog, which correctly stays as-is.

job-tracker-ui/src/components/KanbanBoard.tsx

@@ -158,8 +158,7 @@ export default function KanbanBoard() {
                   minHeight: 220,
                   flex: { xs: "0 0 85vw", md: "none" },
                   scrollSnapAlign: { xs: "start", md: "none" },
-                  border: "1px solid",
-                  borderColor: "divider",
+                  border: "none",
                   backgroundColor: theme.palette.grey[100],
                 }}
               >