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>

account/bp/fragments/routes.py

@@ -9,7 +9,7 @@ Fragments:
 
 from __future__ import annotations
 
-from quart import Blueprint, Response, request, render_template
+from quart import Blueprint, Response, request
 
 from shared.infrastructure.fragments import FRAGMENT_HEADER
 
@@ -22,10 +22,13 @@ def register():
     # ---------------------------------------------------------------
 
     async def _auth_menu():
+        from shared.infrastructure.urls import account_url
+        from shared.sexp.jinja_bridge import sexp as render_sexp
+
         user_email = request.args.get("email", "")
-        return await render_template(
-            "fragments/auth_menu.html",
-            user_email=user_email,
+        return render_sexp(
+            '(~auth-menu :user-email user-email :account-url account-url)',
+            **{"user-email": user_email or None, "account-url": account_url("")},
         )
 
     _handlers = {