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/browser/app/errors.py

@@ -57,6 +57,18 @@ def _error_page(message: str) -> str:
     )
 
 
+def _sexp_error_page(errnum: str, message: str, image: str | None = None) -> str:
+    """Render an error page via s-expressions. Bypasses Jinja entirely."""
+    from shared.sexp.page import render_page
+
+    return render_page(
+        '(~error-page :title title :message message :image image :asset-url "/static")',
+        title=f"{errnum} Error",
+        message=message,
+        image=image,
+    )
+
+
 def errors(app):
     def _info(e):
         return {
@@ -82,10 +94,17 @@ def errors(app):
                 errnum='404'
             )
         else:
-            html = await render_template(
-                "_types/root/exceptions/_.html",
-                errnum='404',
-            )
+            # 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',
+                )
 
         return await make_response(html, 404)
 
@@ -98,10 +117,16 @@ def errors(app):
                 errnum='403'
             )
         else:
-            html = await render_template(
-                "_types/root/exceptions/_.html",
-                errnum='403',
-            )
+            try:
+                html = _sexp_error_page(
+                    "403", "FORBIDDEN",
+                    image="/static/errors/403.gif",
+                )
+            except Exception:
+                html = await render_template(
+                    "_types/root/exceptions/_.html",
+                    errnum='403',
+                )
 
         return await make_response(html, 403)