Fix relations nav_label URL bug and add rich 404 pages with headers

The relations container-nav fragment was inserting nav_label (e.g.
"calendars", "markets") as a URL path segment, generating wrong links
like /the-village-hall/markets/suma/ instead of /the-village-hall/suma/.
The nav_label is for display only, not URL construction.

Also adds a rich 404 handler that shows site headers and post breadcrumb
when a slug can be resolved from the URL path. Falls back gracefully to
the minimal error page if context building fails.

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

shared/browser/app/errors.py

@@ -69,6 +69,107 @@ def _sexp_error_page(errnum: str, message: str, image: str | None = None) -> str
     )
 
 
+async def _rich_error_page(errnum: str, message: str, image: str | None = None) -> str | None:
+    """Try to render an error page with site headers and post breadcrumb.
+
+    Returns HTML string on success, None if we should fall back to the
+    minimal error page.  All failures are swallowed — this must never
+    make a bad situation worse.
+    """
+    from quart import g
+
+    # Skip for internal/static requests
+    if request.path.startswith(("/internal/", "/static/", "/auth/")):
+        return None
+
+    try:
+        # If the app's url_value_preprocessor didn't run (no route match),
+        # manually extract the first path segment as a candidate slug and
+        # try to hydrate post data so context processors can use it.
+        segments = [s for s in request.path.strip("/").split("/") if s]
+        slug = segments[0] if segments else None
+
+        if slug and not getattr(g, "post_data", None):
+            # Set g.post_slug / g.page_slug so app context processors
+            # (inject_post, etc.) can pick it up.
+            from shared.infrastructure.data_client import fetch_data
+            raw = await fetch_data(
+                "blog", "post-by-slug",
+                params={"slug": slug},
+                required=False,
+            )
+            if raw:
+                g.post_slug = slug
+                g.page_slug = slug  # cart uses page_slug
+                g.post_data = {
+                    "post": {
+                        "id": raw["id"],
+                        "title": raw["title"],
+                        "slug": raw["slug"],
+                        "feature_image": raw.get("feature_image"),
+                        "status": raw["status"],
+                        "visibility": raw["visibility"],
+                    },
+                }
+                # Also set page_post for cart app
+                from shared.contracts.dtos import PostDTO, dto_from_dict
+                post_dto = dto_from_dict(PostDTO, raw)
+                if post_dto and getattr(post_dto, "is_page", False):
+                    g.page_post = post_dto
+
+        # Build template context (runs app context processors → fragments)
+        from shared.sexp.page import get_template_context
+        ctx = await get_template_context()
+
+        # Build headers: root header + post header if available
+        from shared.sexp.jinja_bridge import render
+        from shared.sexp.helpers import (
+            root_header_html, call_url, full_page,
+        )
+
+        hdr = root_header_html(ctx)
+
+        post = 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>'
+            )
+
+        # 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>"
+
+        return full_page(ctx, header_rows_html=hdr, content_html=error_html)
+    except Exception:
+        current_app.logger.debug("Rich error page failed, falling back", exc_info=True)
+        return None
+
+
 def errors(app):
     def _info(e):
         return {
@@ -94,17 +195,22 @@ def errors(app):
                 errnum='404'
             )
         else:
-            # Render via s-expressions (Phase 5 proof-of-concept)
-            try:
-                html = _sexp_error_page(
-                    "404", "NOT FOUND",
-                    image="/static/errors/404.gif",
-                )
-            except Exception:
-                html = await render_template(
-                    "_types/root/exceptions/_.html",
-                    errnum='404',
-                )
+            # Try a rich error page with site headers + post breadcrumb
+            html = await _rich_error_page(
+                "404", "NOT FOUND",
+                image="/static/errors/404.gif",
+            )
+            if html is None:
+                try:
+                    html = _sexp_error_page(
+                        "404", "NOT FOUND",
+                        image="/static/errors/404.gif",
+                    )
+                except Exception:
+                    html = await render_template(
+                        "_types/root/exceptions/_.html",
+                        errnum='404',
+                    )
 
         return await make_response(html, 404)