Show selected sub-page name in white next to admin label

Appends e.g. "settings" in white text next to the admin shield icon
on the left side of the admin row, in addition to the highlighted
nav button on the right.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

blog/sexp/sexp_components.py

@@ -99,6 +99,8 @@ def _post_admin_header_html(ctx: dict, *, oob: bool = False, selected: str = "")
 
     admin_href = qurl("blog.post.admin.admin", slug=slug)
     label_html = render("blog-admin-label")
+    if selected:
+        label_html += f' <span class="text-white">{escape(selected)}</span>'
 
     nav_html = _post_admin_nav_html(ctx, selected=selected)