Make post header row generic: admin cog + container_nav in shared helper

Move admin cog generation and container_nav border wrapping from
blog-specific wrapper into shared post_header_html so all services
render identical post header rows. Blog, events, cart all delegate
to the shared helper now. Cart admin pages fetch container_nav_html
via fragments. Village Hall always links to blog.

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

shared/sexp/helpers.py

@@ -89,9 +89,31 @@ def post_header_html(ctx: dict, *, oob: bool = False) -> str:
 
     container_nav = ctx.get("container_nav_html", "")
     if container_nav:
-        nav_parts.append(container_nav)
+        nav_parts.append(
+            '<div class="flex flex-col sm:flex-row sm:items-center gap-2'
+            ' border-r border-stone-200 mr-2 sm:max-w-2xl"'
+            ' id="entries-calendars-nav-wrapper">'
+            f'{container_nav}</div>'
+        )
 
+    # Admin cog — external link to blog admin (generic across all services)
     admin_nav = ctx.get("post_admin_nav_html", "")
+    if not admin_nav:
+        rights = ctx.get("rights") or {}
+        has_admin = rights.get("admin") if isinstance(rights, dict) else getattr(rights, "admin", False)
+        if has_admin and slug:
+            from quart import request
+            admin_href = call_url(ctx, "blog_url", f"/{slug}/admin/")
+            is_admin_page = "/admin" in request.path
+            sel_cls = "!bg-stone-500 !text-white" if is_admin_page else ""
+            base_cls = ("justify-center cursor-pointer flex flex-row"
+                        " items-center gap-2 rounded bg-stone-200 text-black p-3")
+            admin_nav = (
+                f'<div class="relative nav-group">'
+                f'<a href="{escape(admin_href)}"'
+                f' class="{base_cls} {sel_cls}">'
+                f'<i class="fa fa-cog" aria-hidden="true"></i></a></div>'
+            )
     if admin_nav:
         nav_parts.append(admin_nav)