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>

cart/sexp/sexp_components.py

@@ -39,9 +39,32 @@ def _ensure_post_ctx(ctx: dict, page_post: Any) -> dict:
     return ctx
 
 
-def _post_header_html(ctx: dict, page_post: Any, *, oob: bool = False) -> str:
+async def _ensure_container_nav(ctx: dict) -> dict:
+    """Fetch container_nav_html if not already present (for post header row)."""
+    if ctx.get("container_nav_html"):
+        return ctx
+    post = ctx.get("post") or {}
+    post_id = post.get("id")
+    slug = post.get("slug", "")
+    if not post_id:
+        return ctx
+    from shared.infrastructure.fragments import fetch_fragments
+    nav_params = {
+        "container_type": "page",
+        "container_id": str(post_id),
+        "post_slug": slug,
+    }
+    events_nav, market_nav = await fetch_fragments([
+        ("events", "container-nav", nav_params),
+        ("market", "container-nav", nav_params),
+    ], required=False)
+    return {**ctx, "container_nav_html": events_nav + market_nav}
+
+
+async def _post_header_html(ctx: dict, page_post: Any, *, oob: bool = False) -> str:
     """Build post-level header row from page_post DTO, using shared helper."""
     ctx = _ensure_post_ctx(ctx, page_post)
+    ctx = await _ensure_container_nav(ctx)
     return _shared_post_header_html(ctx, oob=oob)
 
 
@@ -794,7 +817,7 @@ async def render_cart_admin_page(ctx: dict, page_post: Any) -> str:
     """Full page: cart page admin overview."""
     content = _cart_admin_main_panel_html(ctx)
     root_hdr = root_header_html(ctx)
-    post_hdr = _post_header_html(ctx, page_post)
+    post_hdr = await _post_header_html(ctx, page_post)
     admin_hdr = _cart_page_admin_header_html(ctx, page_post)
     return full_page(ctx, header_rows_html=root_hdr + post_hdr + admin_hdr, content_html=content)
 
@@ -814,7 +837,7 @@ async def render_cart_payments_page(ctx: dict, page_post: Any) -> str:
     """Full page: payments config."""
     content = _cart_payments_main_panel_html(ctx)
     root_hdr = root_header_html(ctx)
-    post_hdr = _post_header_html(ctx, page_post)
+    post_hdr = await _post_header_html(ctx, page_post)
     admin_hdr = _cart_page_admin_header_html(ctx, page_post, selected="payments")
     return full_page(ctx, header_rows_html=root_hdr + post_hdr + admin_hdr, content_html=content)