Phase 5: Page layouts as s-expressions — components, fragments, error pages

Add 9 new shared s-expression components (cart-mini, auth-menu,
account-nav-item, calendar-entry-nav, calendar-link-nav, market-link-nav,
post-card, base-shell, error-page) and wire them into all fragment route
handlers. 404/403 error pages now render entirely via s-expressions as a
full-page proof-of-concept, with Jinja fallback on failure.

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

shared/sexp/page.py

@@ -0,0 +1,32 @@
+"""
+Full-page s-expression rendering.
+
+Provides ``render_page()`` for rendering a complete HTML page from an
+s-expression, bypassing Jinja entirely.  Used by error handlers and
+(eventually) by route handlers for fully-migrated pages.
+
+Usage::
+
+    from shared.sexp.page import render_page
+
+    html = render_page(
+        '(~error-page :title "Not Found" :message "NOT FOUND" :image img :asset-url aurl)',
+        image="/static/errors/404.gif",
+        asset_url="/static",
+    )
+"""
+
+from __future__ import annotations
+
+from typing import Any
+
+from .jinja_bridge import sexp
+
+
+def render_page(source: str, **kwargs: Any) -> str:
+    """Render a full HTML page from an s-expression string.
+
+    This is a thin wrapper around ``sexp()`` — it exists to make the
+    intent explicit in call sites (rendering a whole page, not a fragment).
+    """
+    return sexp(source, **kwargs)