refactor(helpdesk): use semantic <hr> instead of CSS borders for rule separators

Replace border-bottom on rule items and border-top on advanced disclosure
with native <hr> elements that inherit Core's muted-border styling.
Module CSS now only handles layout, not border presentation.

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

modules/helpdesk/web/css/helpdesk.css

@@ -39,11 +39,10 @@
     gap: 0 calc(var(--app-spacing) * 0.5);
     align-items: center;
     padding: calc(var(--app-spacing) * 0.4) 0;
-    border-bottom: 1px solid color-mix(in srgb, var(--app-contrast, #000) 8%, transparent);
   }
 
-  .helpdesk-settings-rule:last-child {
-    border-bottom: none;
+  .helpdesk-settings-rules > hr {
+    margin: 0;
   }
 
   .helpdesk-settings-rule > input[type="checkbox"] {
@@ -101,8 +100,6 @@
   /* Advanced options disclosure */
   .helpdesk-settings-advanced {
     margin-top: calc(var(--app-spacing) * 0.5);
-    border-top: 1px solid color-mix(in srgb, var(--app-contrast, #000) 8%, transparent);
-    padding-top: calc(var(--app-spacing) * 0.4);
   }
 
   .helpdesk-settings-advanced > summary {