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/browser/app/errors.py

@@ -83,8 +83,7 @@ async def _rich_error_page(errnum: str, message: str, image: str | None = None)
         return None
 
     try:
-        from shared.sexp.jinja_bridge import render
-        from shared.sexp.helpers import full_page, call_url
+        from shared.sexp.helpers import full_page
 
         # Build a minimal context — avoid get_template_context() which
         # calls cross-service fragment fetches that may fail.
@@ -150,44 +149,22 @@ async def _rich_error_page(errnum: str, message: str, image: str | None = None)
                 pass
 
         # Root header (site nav bar)
-        from shared.sexp.helpers import root_header_html
+        from shared.sexp.helpers import (
+            root_header_html, post_header_html,
+            header_child_html, error_content_html,
+        )
         hdr = root_header_html(ctx)
 
         # Post breadcrumb if we resolved a post
         post = (post_data or {}).get("post") or ctx.get("post") or {}
         if post.get("slug"):
-            title = (post.get("title") or "")[:160]
-            feature_image = post.get("feature_image")
-            label_html = ""
-            if feature_image:
-                label_html += (
-                    f'<img src="{feature_image}" '
-                    f'class="h-8 w-8 rounded-full object-cover border border-stone-300 flex-shrink-0">'
-                )
-            label_html += f"<span>{escape(title)}</span>"
-            post_row = render(
-                "menu-row",
-                id="post-row", level=1,
-                link_href=call_url(ctx, "blog_url", f"/{post['slug']}/"),
-                link_label_html=label_html,
-                child_id="post-header-child",
-                external=True,
-            )
-            hdr += (
-                f'<div id="root-header-child" class="w-full">'
-                f'{post_row}'
-                f'</div>'
-            )
+            ctx["post"] = post
+            post_row = post_header_html(ctx)
+            if post_row:
+                hdr += header_child_html(post_row)
 
         # Error content
-        error_html = (
-            '<div class="text-center p-8 max-w-lg mx-auto">'
-            f'<div class="font-bold text-2xl md:text-4xl text-red-500 mb-4">{errnum}</div>'
-            f'<div class="text-stone-600 mb-4">{escape(message)}</div>'
-        )
-        if image:
-            error_html += f'<div class="flex justify-center"><img src="{image}" width="300" height="300"></div>'
-        error_html += "</div>"
+        error_html = error_content_html(errnum, message, image)
 
         return full_page(ctx, header_rows_html=hdr, content_html=error_html)
     except Exception:

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 = "",

shared/sexp/templates/layout.sexp

@@ -146,6 +146,32 @@
         (div :id child-id :class "flex flex-col w-full items-center"
           (when child-html (raw! child-html)))))))
 
+(defcomp ~post-label (&key feature-image title)
+  (<> (when feature-image
+        (img :src feature-image :class "h-8 w-8 rounded-full object-cover border border-stone-300 flex-shrink-0"))
+    (span title)))
+
+(defcomp ~page-cart-badge (&key href count)
+  (a :href href :class "relative inline-flex items-center gap-1.5 px-3 py-1.5 text-sm rounded-full border border-emerald-300 bg-emerald-50 text-emerald-800 hover:bg-emerald-100 transition"
+    (i :class "fa fa-shopping-cart" :aria-hidden "true")
+    (span count)))
+
+(defcomp ~oob-header (&key parent-id child-id row-html)
+  (div :id parent-id :hx-swap-oob "outerHTML" :class "w-full"
+    (div :class "w-full" (raw! row-html)
+      (div :id child-id))))
+
+(defcomp ~header-child (&key id inner-html)
+  (div :id (or id "root-header-child") :class "w-full" (raw! inner-html)))
+
+(defcomp ~error-content (&key errnum message image)
+  (div :class "text-center p-8 max-w-lg mx-auto"
+    (div :class "font-bold text-2xl md:text-4xl text-red-500 mb-4" errnum)
+    (div :class "text-stone-600 mb-4" message)
+    (when image
+      (div :class "flex justify-center"
+        (img :src image :width "300" :height "300")))))
+
 (defcomp ~nav-link (&key href hx-select label icon aclass select-colours is-selected)
   (div :class "relative nav-group"
     (a :href href