fix(helpdesk): prevent vertical text in table headers on narrow viewports

Add white-space:nowrap + overflow:hidden on th elements. Set min-width
on table so columns never shrink below readable size — scroll wrapper
handles the overflow.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

modules/helpdesk/web/css/helpdesk.css

@@ -196,6 +196,7 @@
     border-spacing: 0;
     overflow: hidden;
     table-layout: fixed;
+    min-width: 50rem;
   }
 
   /* Column widths — consistent across all agent cards */
@@ -211,6 +212,9 @@
     padding: calc(var(--app-spacing) * 0.3) calc(var(--app-spacing) * 0.5);
     border-bottom: 1px solid var(--app-muted-border-color);
     background: color-mix(in srgb, var(--app-muted-border-color) 20%, transparent);
+    white-space: nowrap;
+    overflow: hidden;
+    text-overflow: ellipsis;
   }
 
   .helpdesk-team-ticket-table td {