Consolidate post header/menu system into shared infrastructure

Replace duplicated _post_header_html, _oob_header_html, and header-child
components across blog/events/market/errors with shared sexpr components
(~post-label, ~page-cart-badge, ~oob-header, ~header-child, ~error-content)
and shared Python helpers (post_header_html, oob_header_html,
header_child_html, error_content_html). App-specific logic (blog container-nav
wrapping, admin cog, events calendar links) preserved via thin wrappers.

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

shared/sexp/helpers.py

@@ -67,6 +67,59 @@ def search_desktop_html(ctx: dict) -> str:
     )
 
 
+def post_header_html(ctx: dict, *, oob: bool = False) -> str:
+    """Build the post-level header row (level 1). Used by all apps + error pages."""
+    post = ctx.get("post") or {}
+    slug = post.get("slug", "")
+    if not slug:
+        return ""
+    title = (post.get("title") or "")[:160]
+    feature_image = post.get("feature_image")
+
+    label_html = render("post-label", feature_image=feature_image, title=title)
+
+    nav_parts: list[str] = []
+    page_cart_count = ctx.get("page_cart_count", 0)
+    if page_cart_count and page_cart_count > 0:
+        cart_href = call_url(ctx, "cart_url", f"/{slug}/")
+        nav_parts.append(render("page-cart-badge", href=cart_href, count=str(page_cart_count)))
+
+    container_nav = ctx.get("container_nav_html", "")
+    if container_nav:
+        nav_parts.append(container_nav)
+
+    admin_nav = ctx.get("post_admin_nav_html", "")
+    if admin_nav:
+        nav_parts.append(admin_nav)
+
+    nav_html = "".join(nav_parts)
+    link_href = call_url(ctx, "blog_url", f"/{slug}/")
+
+    return render("menu-row",
+        id="post-row", level=1,
+        link_href=link_href, link_label_html=label_html,
+        nav_html=nav_html, child_id="post-header-child",
+        oob=oob, external=True,
+    )
+
+
+def oob_header_html(parent_id: str, child_id: str, row_html: str) -> str:
+    """Wrap a header row in an OOB swap div with child placeholder."""
+    return render("oob-header",
+        parent_id=parent_id, child_id=child_id, row_html=row_html,
+    )
+
+
+def header_child_html(inner_html: str, *, id: str = "root-header-child") -> str:
+    """Wrap inner HTML in a header-child div."""
+    return render("header-child", id=id, inner_html=inner_html)
+
+
+def error_content_html(errnum: str, message: str, image: str | None = None) -> str:
+    """Render the error content block."""
+    return render("error-content", errnum=errnum, message=message, image=image)
+
+
 def full_page(ctx: dict, *, header_rows_html: str,
               filter_html: str = "", aside_html: str = "",
               content_html: str = "", menu_html: str = "",