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: